PROJECT TITLE:
Fundamental Investigations of ClO2 Delignification –
Final Report
PROJECT #:
DE-FC07-96ID13442
PROJECT SPONSOR:
United States Department of Energy
Agenda 2020 Environmental Performance
A Technology Vision & Research Agenda
For America’s Forest, Wood, and Paper Industry
PRINCIPAL INVESTIGATORS:
1.
Dr. A.J. Ragauskas, Professor of Engineering, Institute of Paper Science and
Technology, 500 10th St., NW, Atlanta GA 30318
1.1 INTRODUCTION:
Over the last decade environmental regulations1,2
3,4
and renewed consumer activism have substantially impacted many pulp and paper
operations. Among the many chemical processes involved in the manufacturing of paper,
few processes have drawn as much environmental attention as the bleaching of chemical
pulps. Beginning with the detection of polychlorinated dibenzo-p-dioxins and
dibenzofurans in kraft bleach plant effluents, the pulp and paper industry has been
required to address a series of environmental performance issues.5,6 To address these
issues the pulp and paper industry has begun to implement a series of process changes
including extending the pulping process to yield kraft pulps with low lignin content,
improved brownstock washing, use of oxygen delignification technologies, 100%
chlorine dioxide substitution, and fortification of extraction stages with oxygen and
hydrogen peroxide.7,8,9,10 Based upon a variety of performance and environmental
considerations, chlorine dioxide remains one of the most attractive bleaching agents for
kraft pulp.11,12
Recent research results from the principal investigator’s laboratories have shown that
although free phenolic structures are important in ClO2 bleaching chemistry, their content
in unbleached kraft pulps cannot simply be related to the observed bleaching response.13
Lower kappa number pulps were found to contain a greater content of free phenolic
structures, yet bleaching experiments in general show that as the kappa number is
decreased there is a slight increase in total available chlorine (TAC) consumption per unit
kappa number reduction in the D(EO) sequence. Reactivity studies of ClO2 with residual
lignin suggest that residual lignin in higher kappa number pulp is more reactive towards
ClO2, which is consistent with the observed bleaching response. The goal of the present
project was to identify important structural components of kraft pulps that control pulp
bleachability and employ this knowledge to develop strategies to improve future
bleaching operations.
1.2 PROJECT OBJECTIVES:
The overall objective of this project was to develop
a fundamental understanding of the mechanisms of chlorine dioxide delignification of
low kappa kraft pulps and identify new methods of improving the efficiency and
effectiveness of this bleaching agent. The approach adopted was to investigate the
fundamental structural components of lignin that contribute to delignification reactions
with chlorine dioxide. These results were then used to examine new bleaching
technologies that will permit enhanced delignification while simultaneously reducing the
generation of chlorinated organic compounds.
1.3 RESEARCH JUSTIFICATION:
Efficient delignification during ECF bleaching
is believed to be significantly influenced by the reactivity of residual lignin towards chlorine
dioxide. The structure of residual lignin is believed to play a key role in this reactivity,
particularly the content of free phenolic groups, which have already been shown to have an
effect on ClO2.
Studies by Froass, et al.13 began to examine the fundamental chemical basis for these
relationships. In this preliminary investigation, we studied the ClO2 delignification of five
pulps. These were conventional kraft (CK) pulps having unbleached kappa numbers of 28.0
and 18.5 and simulated EMCC™ pulps having kappa numbers of 29.1, 18.5, and 14.5 (see
Table 1.1).
Table 1.1. Bleachability parameters for conventional and EMCC™ pulps.
Parameter
Conventional
EMCC™
Unbleached Kappa No. 28
18.5
29.1 18.5 14.5
a
Kappa No. after D
11.0
9.6
11.0 7.5
6.0
TAC/ Kappa
0.30
0.38
0.30 0.31 0.33
b
Kappa No. after (EO)
3.8
3.8
3.3
3.0
2.3
TAC/ Kappa after (EO) 0.21
0.23
0.21 0.22 0.23
Brightness after (EO)
47.4
49.9
49.8 50.2 53.5
a
o
D0 was at 45 C, 10% consistency, 30 min., kappa factor of 0.187;b (EO): 70oC, 10%
consistency, 60 min., NaOH charge of 50% of TAC charge, O2 press. was initially 60 psi
and decreased by 12 psi/5 min.
The results of this study indicated that decreasing the unbleached kappa number causes an
increase in ClO2 consumption per unit of kappa number reduction in both the D and D(EO)
stages. At a given unbleached kappa no., EMCC™ is more readily delignified in the D(EO)
stages than CK, resulting in a lower overall chemical requirement for EMCC™.
The significance of these results in the present context is that they established the existence
of differences in ClO2 bleachability between pulps, in particular between pulps of similar
kappa numbers, and provided an incentive to implement methods for residual lignin
structural analysis. These are the same methods that are most appropriate for examining the
mechanistic pathways that determine the course of ClO2 delignification.
Residual lignin was isolated from each of the kraft pulps described in Table 1.1 and analyzed
by NMR spectroscopy. 13C NMR analysis indicated that as the kappa number is decreased
for both conventional and EMCC™ pulps there is a decrease in etherified linkages in the
residual lignin structure, most of which can probably be assigned to aryl ether linkages. The
results also show that more substituted aromatic carbons are found in the residual lignins as
the kappa number is decreased. This result was substantiated by derivatizing the residual
lignin samples isolated from the conventional and EMCC™ kraft pulps with a selective
phosphitylation reagent and examining the derivatized material by means of 31P NMR. The
31
P-spectra indicated that concentration of condensed phenolic units in the residual lignin
increased as kappa number decreased. The 31P NMR data also suggested that the phenolic
content of the EMCC™ residual lignins continued to increase with decreasing kappa number
while for the CK residual lignins the phenolic content leveled off after kappa number of 18
was reached. At low kappa numbers the EMCC™ residual lignin contains more phenolic
structures (both condensed and uncondensed) than the CK residual lignin. Although the
increased phenolic content of the former correlates with its easier bleachability, the
relationship is not a simple one, since the lower kappa number pulps, which were more
difficult to bleach, had a higher content of phenolic structures. Structural analysis revealed
that the higher kappa number pulps had a higher content of aryl ether linkages and a lower
content of condensed lignin structures. This suggests that such a lignin may be more
reactive towards ClO2. In summary, work prior to this project indicated that the reactivity of
ClO2 towards conventional and EMCC™ kraft pulps may be related to residual lignin
structure. A goal of the current project was to examine this relationship on a broader set of
pulps and to determine if these relationships are global to SW and HW kraft pulps.
1.4 SUMMARY OF PREVIOUS WORK: In summary, the bleachability of
conventional and EMCC® kraft pulps in a D(EO) sequence was related to the residual
lignin structure and its reactivity towards ClO2. During the delignification stages (D and
D(EO)) the ClO2 consumption per unit kappa number reduction increased slightly as the
kappa numbers of both the CK and EMCC® pulps decreased. This result suggests that the
higher kappa number pulps contain residual lignin that is more reactive towards the ClO2.
Analysis of the residual lignin structure suggested that the content of free phenolic
structures could not be simply related to this result, since the lower kappa number pulps
had a higher content of phenolic structures. The structural analysis revealed that the
higher kappa number pulps had a higher content of aryl ether linkages and a lower
content of condensed lignin structures, possibly including condensed nonphenolic
structures. The higher content of aryl ether linkages and lower amounts of condensed
structure suggests that such a lignin may be more reactive towards ClO2. Experiments in
which the isolated residual lignins were reacted with ClO2 showed that the higher kappa
number residual lignins, in spite of their lower phenolic content, had a larger increase in
carboxylic acid formation, suggesting a more reactive lignin. The bleachability
experiments also showed that EMCC® pulps were more readily delignified in the D(EO)
stages than the CK pulps, resulting in a slightly lower overall chemical requirement for
EMCC®. The residual lignin reactivity experiments suggested that the EMCC® pulp
residual lignin was slightly more reactive than the CK residual lignin, which is consistent
with the bleachability result.
1.5 PROJECT OBJECTIVES:
The goals of this program were to establish the
important structures in pulp that influence ClO2 pulp bleachability as it applies to D(EO)
pulp delignification and to employ the results to improve the relevant pulping and
bleaching technologies.
2.0
PARAMETERS THAT INFLUENCE THE ClO2 BLEACHABILITY OF SW
KRAFT PULPS
2.1
INTRODUCTION:
As reviewed in the introduction section, the bleachability of kraft pulps appears to be influenced
by a variety of factors including the structure of residual lignin. Lignin is a complex polymer in
wood that contributes approximately 25-30% of the mass of wood. During pulping this polymer
is selectively degraded; near the end of a kraft cook the selectivity of alkaline degradation
between pulp carbohydrates and lignin decreases dramatically, and the cook must be halted to
prevent loss of carbohydrates. At the completion of the kraft pulping process for bleachable
grades, typically 3-5% lignin remains. For SW kraft pulps residual lignin consists of several
lignin functional groups derived from guaiacyl units; Table 2.1 summarizes some of the
predominant functional groups known to be present.
Table 2.1.
lignin.
Description of predominant functional groups present in SW kraft residual
Lignin Functional Unit
Noncondensed C5
phenoxy groups
Structure
OCH3
OH
Noncondensed C5
phenoxy groups
R: Ph, -CH 2Ph
R
OCH 3
OH
Methoxy groups
β-O-aryl ether
Ph-Ome
HO
O
OH
OMe
OMe
OH
Enol Ether
O
OMe
OMe
OH
Carboxyl groups
RCO2H
Advances in pulping technology have continued unabated for the last decade in response to
environmental and market pressures. The ability to lower the content of lignin in kraft pulps
prior to bleaching facilitates environmentally compatible bleaching practices, reduces operating
costs, and may assist in the development of low-effluent pulp production. The fundamental
principles involved in extending kraft delignification while retaining pulp strength properties
were established in the late 1970s and early 1980s.14,15,16 These principles include leveling out
the alkali concentration, maintaining a high sulfidity particularly at the beginning of the cook,
reducing the dissolved lignin concentration, and employing lower cooking temperatures.
Recently, further improvements have been made in continuous modified kraft cooking
technology to enhance delignification while improving pulp strength and uniformity. 17,18
Despite these significant advances in pulping technology, very little is known about how changes
in the kraft pulping process can influence residual lignin structure. To address this issue, several
researchers have begun to characterize the nature of residual lignin in kraft pulps.19,20,21,22,23,24
As a preliminary study in this field, we have previously reported that the functional groups of
lignin can be influenced by the extent of delignification and type of pulping process employed.25
In this study, we examined the fundamental changes in lignin structure that occur when loblolly
pine is cooked under conventional and simulated Lo-Solids® kraft pulping conditions. LoSolids® is representative of the next generation of extended modified continuous cooking
technologies and can be readily retrofitted to EMCC™ kraft digesters. A description of the SW
furnishes employed and cooking parameters used is presented in the experimental section (2.4)
of this report.
Prior research studies by Froass et al.25 certainly suggested that the extent of delignification and
kraft cooking technology could influence bleachability. Although these results were supported
by literature results, it was unanticipated that pulp bleachability of a D(EO) treatment would not
be directly related to the amounts of phenoxy groups present in the lignin. This result was
significant since most bleaching textbooks26 assert that the ClO2 bleaching chemistry occurs
primarily via phenoxy groups and that this unit in lignin controls pulp bleachability. To define
the influence of kraft pulping on ClO2 bleaching of SW kraft pulps, a series of conventional and
extended modified continuous kraft pulps were prepared, analyzed, and bleached via D(EO).
The changes in lignin structure after pulping and bleaching were evaluated to determine if lignin
structure contributed to pulp bleachability.
2.2
RESULTS AND DISCUSSION:
2.21
Cooking Results
To minimize experimental variations due to the wood source, a single wood sample was used for
all the studies described in this paper. To fully explore the dependency of lignin structure on
kraft pulping process type, we elected to prepare a series of conventional (CK) and Lo-Solids®
(LS) kraft pulps covering a kappa number range of 39.3-11.0. Table 2.2 provides a brief
description of the physical properties of these kraft pulps. These data clearly demonstrate that
extended modified continuous cooking procedures can attain lower lignin content pulps while
retaining higher viscosity values.
Table 2.2.
Physical properties and sample abbreviations for SW kraft pulps.
Lo-Solids® Kraft Pulps
Sample Kappa # Viscosity
#
mPa
LS29
29.3
43.4
LS26
26.1
36.1
LS19
19.1
25.5
LS17
17.1
21.1
LS16
16.0
19.2
LS11
11.0
12.1
Conventional Kraft Pulps
Sample Kappa #
Viscosity
#
mPa
CK33
33.0
32.6
CK28
28.3
24.8
CK21
21.3
22.6
CK20
20.4
22.4
CK18
18.1
18.5
CK15
14.7
13.3
CK13
13.3
13.1
2.12
Kraft Pulps - Analyzed by 1H NMR
Our interest in these pulps was twofold. First, we were interested in determining if pulps of
differing lignin contents and kraft cooking technologies would have differing types of lignin
structures. Second, we wished to determine if the lignin structures present in the LK and CK
pulps would also impact pulp bleachability. To examine these issues, the residual lignin from
kraft pulps CK33-13 and LS29-11 were isolated and characterized by NMR. Following
literature methods, residual lignin was extracted from each of these pulps employing an acidic
dioxane solution. After purification, typical yields were in the range of 45-65%. The lignin
isolated from the kraft pulps was then characterized by employing quantitative 1H NMR methods
that have been developed by Lundquist.39 Figure 2.1 provides a representative spectrum of a
lignin sample analyzed by proton NMR.
Methoxy H
PFB
Aromatic H
Aliphatic H
Form ylH
Phenolic OH
COOH
12
10
8
6
4
2
0
(ppm )
Figure 2.1. Quantitative 1H NMR spectrum of lignin samples.
The analysis of lignin structure by NMR is well described in the literature and provides a facile
means for characterizing a variety of functional groups of lignin, including acid groups,
methoxyl, phenoxy, and aliphatic units. The results of these analyses for the 13 pulps studied
in this paper are summarized in Figures 2.2-2.5.
16 mmol of H/g of Lignin
14
12
10
8
6
4
2
0
10
15
CK
CKth
l
CK guaiacyl
ti
h
li
20
Kappa
N b
25
30
35
CK
CK
li h5-sub
ti guaiacyl
CK
h
li
COOH
mmol of H/g of Lignin
Figure 2.2. Content of different structural moieties in lignin samples isolated from CK
pulps as determined by 1H NMR.
16
14
12
10
8
6
4
2
0
10.0
15.0
LS methoxyl
LS aromatic
LS guaiacyl phenolic
20.0
Kappa Number
25.0
30.0
LS aliphatic
LS 5-sub guaiacyl phenolic
LS COOH
Figure 2.3. Content of different structural moieties in lignin samples isolated from LS
pulps as determined by 1H NMR.
It is clear from these studies that for both conventional (CK) and modified (LS) kraft pulps the
content of methoxyl and aromatic hydrogens in residual lignin decreases as the extent of
delignification is increased. For the CK pulps, the aliphatic and guaiacyl units also decrease as
delignification proceeds. In contrast, for the LS pulps, the aliphatic and guaiacyl units appear to
be at a minimum value for the LS17 pulp. The lignin acid groups for both the CK and LS pulps
remain approximately constant.
mmol of H/g of Lignin
A comparison of the structural groups of residual lignins from the LS and CK kraft pulps
provides additional information on the nature of kraft pulping and the influence that process
modifications can have on the final product.
19
17
15
13
11
LS methoxyl
9
CK methoxyl
7
5
10
15
20
25
30
35
Kappa Number
Figure 2.4. Changes in methoxyl content of residual lignin samples isolated from LS and
CK pulps as determined by 1H NMR.
mmol of H/g of Lignin
1.6
1.5
1.4
1.3
LS guaiacyl phenolic
1.2
CK guaiacyl phenolic
1.1
1.0
0.9
0.8
10
15
20
25
30
35
Kappa Number
Figure 2.5. Changes in guaiacyl content of residual lignin samples isolated from LS and
CK pulps as determined by 1H NMR.
As shown in Figure 2.4, the content of methoxyl groups was found to be lower in the modified
kraft than in the conventional kraft pulps. The guaiacyl and 5-substituted guaiacyl units of the
conventional and modified kraft pulps (see Fig. 2.5) exhibited a more complex pattern. At high
kappa values, both the conventional and modified kraft pulps (ca. 30 kappa number) have
comparable levels of phenoxy groups, but as delignification is extended, the LS pulps exhibit a
decrease in free phenols whereas the conventional have an increase. Finally, at low kappa levels
(<15), the extended modified pulps appear to have higher contents of free phenols. We have
previously noted25 a comparable pattern of residual lignin when comparing a conventional kraft
pulp with an EMCC pulp. These differences in lignin structure were attributed to differing
amounts of β-O-aryl ethers present in the pulp as delignification proceeds due to the differences
in pulping.
mmol of H/g of Lignin
15
13
11
LS aromatic
CK aromatic
9
7
5
10
15
20
25
30
35
Kappa Number
Figure 2.6. Changes in aromatic proton content of residual lignin samples isolated from LS
and CK pulps as determined by 1H NMR.
Figure 2.6 shows that there is a decrease in aromatic proton character in residual lignin as
delignification is increased from the kappa ∼30 pulps to kappa 11 pulps. Furthermore, as
previously noted, the extended modified pulps appear to have lower amounts of aromatic protons
than the conventional pulps, suggesting that they are further substituted. These results again
mirror our previous results.25
2.13
Kraft Pulps - Analyzed by 31P NMR
The characterization of lignin by derivatization with 2-chloro-4,4,5,5-tetramethyl-1,3,2dioxaphospholane (see Eq. 1) followed by analysis by 31P NMR allows for facile analysis of the
hydroxy groups present in lignin.
Me
Me
Me
Me
O
O
P
Eq. 1)
R
Cl
OH
R=lignin
RO
O
Me
Me
O
Me
Me
P
Fig. 2.7 provides typical 31P NMR spectral data acquired from residual lignin. This approach
facilitates the measurement of aliphatic, condensed, and noncondensed phenolics and carboxylic
acid groups. Figures 2.8-2.11 summarize the changes in lignin structure between conventional and
Lo-Solids® kraft softwood pulps at varying levels of lignin content.
ppm
150
148
146
Aliphatic
OH
Figure 2.7.
31
144
142
140
Cyclohexanol Condensed
phenolic
138
136
134
Carboxyl
groups
Guaiacyl
phenolic
P NMR spectral data for phosphitylated residual lignin.
mmol of OH/g of lignin
3
2.5
CK
CK
CK
CK
2
1.5
1
0.5
0
10
15
20
25
30
35
Kappa Number
Figure 2.8. Content of aliphatic, guaiacyl, condensed guaiacyl, and carboxylic hydroxy
groups in lignin samples isolated from CK pulps as determined by 31P NMR.
mmol of OH/g of lignin
2.5
2
LS Aliphatic
1.5
LS Condensed
LS Guaiacyl
1
LS COOH
0.5
0
10
15
20
25
30
Kappa Number
Figure 2.9. Content of aliphatic, guaiacyl, condensed guaiacyl and carboxylic hydroxy
groups in lignin samples isolated from LS pulps as determined by 31P NMR.
mmol of OH/g of Lignin
2.7
2.5
2.3
2.1
1.9
LS Aliphatic
1.7
CK Aliphatic
1.5
1.3
1.1
0.9
10
15
20
25
Kappa Number
30
35
Figure 2.10. Changes in aliphatic hydroxy content of residual lignin samples isolated from
LS and CK pulps as determined by 31P NMR.
As shown in Figure 2.10, both conventional and Lo-Solids® pulps exhibit a loss in aliphatic signals
as the amount of residual lignin is decreased. The principal mechanism for loss of aliphatic hydroxy
groups is by loss of formaldehyde (see Eq. 2). Certainly, it is to be expected that this reaction would
occur more frequently as delignification proceeds and this is exactly what is observed in Fig. 2.10.
Interestingly, the Lo-Solids® pulps exhibit slightly less aliphatic character, suggesting that this
chemistry is occurring more frequently in the extended modified kraft cooking process.
H3CO
HO
H3CO
O
O
HO
NaOH
Eq. 2
OCH3
OCH3
OH
OH
The changes in acid groups for the CK and LS residual lignin samples are summarized in Figure
2.11.
mmol of OH/g of Lignin
0.55
0.5
0.45
LS COOH
CK COOH
0.4
0.35
0.3
0.25
10
15
20
25
30
35
Kappa Number
Figure 2.11. Changes in acid group content of residual lignin samples isolated from LS and
CK pulps as determined by 31P NMR.
As previously reported,13 conventional pulps exhibit an increase in acid character as
delignification proceeds. To date, we have interpreted the presence of acid groups to be due to
an enrichment of carboxylic acid groups in residual lignin and not due to an oxidative process.
Based on this assumption, it appears that the Lo-Solids® pulping process does not lead to as
significant an accumulation of acid groups as observed in conventional pulping.
mmol of OH/g of Lignin
1.7
1.5
1.3
LS Guaiacyl
1.1
CK Guaiacyl
0.9
0.7
10
15
20
25
Kappa Number
30
35
Figure 2.12. Changes in guaiacyl phenoxy content of residual lignin samples isolated from
LS and CK pulps as determined by 31P NMR.
mmol of OH/g of Lignin
1.5
1.4
1.3
1.2
1.1
1
LS Condensed
0.9
CK Condensed
0.8
0.7
0.6
10
15
20
25
Kappa Number
30
35
Figure 2.13. Changes in condensed guaiacyl phenoxy content of residual lignin samples
isolated from LS and CK pulps as determined by 31P NMR.
Figures 2.12 and 2.13 summarize the analysis of phenoxy groups present in the residual lignin.
Although the results do not exactly mirror the analysis by 1H NMR and we did not expect the
results to be identical, it is clear that the trends in each analysis are comparable. The most
important observation to come out of this analysis is that the Lo-Solids® pulps exhibit a lower
amount of free phenolics (both condensed and noncondensed) than conventional pulps. This
result is very significant given the accepted mechanisms by which chlorine dioxide degrades
lignin via free phenolics.
Based on the 31P and 1H NMR studies it appears that the Lo-Solids® pulping process yields
lignin with less condensed phenolic lignin structure but is overall more condensed than is
typically present in a conventional kraft pulp of comparable lignin content. This result is very
significant since the process changes implemented in Lo-Solids® kraft pulping were targeted, in
part, to minimize condensed lignin and yet we have noted increases in this type of lignin. The
results of these studies have been discussed with Ahlstrom Machinery Corp. and other industry
representatives, and they are evaluating the practical implications of these studies for improved
digester design.
2.2
D(EO) SW KRAFT PULP BLEACHABILITY STUDIES:
From a mill perspective, it is well known that the bleachability of kraft pulps can vary
substantially. Pulp bleachability can be influenced by a wide array of factors from process
variables such as the efficiency of pulp washing27 to fundamental chemistry issues, including the
structure of residual lignin28 and carbohydrates in the fiber.29
Teder and Sjostrom30 have demonstrated that alkaline sulfite pulping yields pulps with
significantly improved bleachability properties with respect to kraft pulps. Classical studies by
Germgard31 have shown that post-oxygen delignified kraft pulps are less bleachable than pre-O2
kraft pulps with chlorine dioxide. Recently, studies by Buchert et al. 32, 33 have advanced our
knowledge on how carbohydrates, specifically hexenuronic acids, can impact pulp bleachability.
It is also well known that the structure of residual lignin can influence pulp bleachability,
especially with chlorine dioxide. This dependency is due in part to the reactivity of chlorine
dioxide with phenoxy groups. Kumar et al.34 studied differences in pulp bleachability between
RDH and conventional kraft pulps and suggested a correlation between phenoxy content of the
residual lignin and bleachability. Gellerstedt and Al-Dajani35 have reported that the bleachability
of softwood kraft pulps with chlorine dioxide or hydrogen peroxide was influenced by kraft
cooking conditions. These differences in bleachability were attributed to differences in residual
lignin content, lignin-carbohydrate complexes, and condensed carbohydrates. Froass et al.25
observed that for softwood kraft pulps, the kappa number of the pulp and the pulping process
employed influenced bleachability. The total available chlorine (TAC) consumed per unit kappa
number reduction in the D(EO) sequence was found to decrease as the kappa number decreased
despite the fact that the lower kappa pulps were found to have higher amounts of phenoxy
groups. A closer examination of the residual lignin structure prior to bleaching suggested that
the amounts of condensed phenolics increased as delignification was extended and it was
hypothesized that these units were detrimentally influencing pulp bleachability.
In this project, we further examine the relationship between pulp bleachability and chlorine
dioxide delignification for a series of conventional pulps and pulps from Lo-Solids® kraft cooks.
The bleachability properties of these pulps were examined after D0 and D(EO). The D0 stage
was performed using a kappa factor of 0.05 and 0.20 at 10% consistency with high shear mixing
and a ClO2 retention time of 1 min. Previous studies by Schwantes and McDonough36 have
shown that substantial delignification of kraft pulps can be accomplished with reaction times
significantly less than what is employed in a typical D0 stage. The resulting pulps were then
alkaline extracted in an oxygen-reinforced stage. Pulp bleachability, measured as TAC/∆(kappa
number), was determined after the D0 and (EO) stage. Residual lignin was isolated before and
after each stage and changes in lignin functional groups were determined employing modern
NMR methods.
2.30
RESULTS AND DISCUSSION:
Bleachability of Kraft Pulps
Recent studies at IPST have shown that the rapid kinetics of chlorine dioxide delignification of
kraft pulps can be utilized for bleaching provided that good uniform mixing can be
accomplished. Preliminary studies reported by Schwantes36 demonstrated that most of the
delignification of a D0 stage is accomplished within the first 1-2 minutes of bleaching, provided
that the bleaching chemical is uniformly distributed throughout the bleaching reactor. This type
of very short bleaching stage, referred to as Rapid D0, reduces AOX formation and offers
significant savings in bleach plant capital equipment. This report examines the results of
applying this process in the delignification of a series of softwood kraft pulps manufactured
under conventional conditions and conditions that correspond to Lo-Solids® cooking. This was
of interest because the short retention time is expected to yield bleached pulps having lignins
whose structural features are unaffected by secondary reactions.
The results summarized in Table 2.3 indicate that the conventional pulps exhibited only slightly
improved bleachability at higher lignin contents when the ClO2 retention time is kept short. The
LS pulps were equally bleachable at high and low lignin contents under these conditions. These
results differ somewhat from those of earlier studies by our group,25 indicating that the decreased
bleachability of low-kappa pulps at normal D0 retention times is related to inhibition of reactions
that occur after the initial phase.
Table 2.3. Bleachability of CK and LS pulps towards Rapid D0 and EO.
Pulp
k.f. TAC
CK-32.4 0.05 1.62
CK-32.4 0.20 6.48
CK-21.3 0.20 4.26
CK-14.7 0.20 2.94
LS-29.3
0.05 1.46
LS-29.3
0.20 5.86
LS-19.1
0.20 3.82
LS-15.6
0.20 3.12
∆ Kappa
D - 5.9
EO-13.6
D - 15.3
EO-24.3
D - 9.2
EO-15.3
D - 6.5
EO-10.6
D - 5.1
EO-13.0
D - 13.7
EO-22.2
D - 8.8
EO-14.6
D - 6.9
EO-11.8
TAC/∆ Kappa
0.274
0.119
0.423
0.267
0.463
0.278
0.452
0.277
0.286
0.112
0.428
0.264
0.434
0.262
0.452
0.264
The LS pulps were modestly easier to delignify than the conventional pulps, requiring a ClO2
charge that was an average of 4% lower than in the case of CK for a given kappa number
reduction measured after the (EO) stage. The TAC/∆kappa values were larger than normally
observed at normal retention times in the D0 stage because no credit was taken for the unutilized
residual remaining after only one minute. In order to limit the D0 reaction time to exactly one
minute, it was necessary to stop the reaction by injection of sulfite, which destroyed the residual
and precluded its measurement. It is for this reason that, in practical applications, low kappa
factors are recommended for Rapid D0.
2.31
Residual Lignin Analysis
The difference in bleachability between the CK and LS pulps suggests that it may be possible to
tailor pulp properties to a bleaching chemical. However, before such a strategy can be rationally
designed, the fundamental principles involved in bleachability must be determined. Several
researchers have begun elucidating how process changes in the kraft pulping can moderate the
structure of residual lignin. Studies by Hortling et al.37 have compared the differences in residual
lignin that occur between Superbatch and conventional kraft pulps. Gellerstedt24 has reported
differences in bleachability of Scandinavian softwood kraft pulps dependent upon how they are
cooked. Jiang and Argyropoulos38 have found that residual lignin from modified continuous kraft
pulping procedures has lower amounts of condensed phenolic units and higher amounts of
carboxylic acids as compared to lignin isolated from conventional kraft pulps. Froass et al. have
shown that pulps prepared under simulated conventional and EMCC® conditions exhibited
differences in residual lignin structure and bleachability.25 It was hypothesized in these latter
studies that condensed phenoxy groups and β-O-aryl ether groups may influence chlorine
dioxide bleachability of kraft pulps. This report further examined the relationship between pulp
bleachability and residual lignin structure.
In section, 2.12 and 2.13 we examined the structure of the residual lignin present in the CK and
LS pulps and demonstrated clear differences in residual lignin structure. The results of these
studies suggested the CK and LS pulps had comparable lignin functional groups at kappa
numbers of approximately 30. As delignification was extended, the CK pulps were found to
have higher amounts of phenolics and methoxyl groups than the LS pulps. To further explore
the relationship between bleachability and residual lignin structure, we isolated residual lignin
structure after the Rapid D0 and (EO) stages employed in the current study. These samples were
then analyzed employing 1H NMR according to the method of Li and Lundquist.39 This
procedure provides a convenient method of determining the concentration of phenolic units,
carboxylic acids, and methoxyl groups in residual lignin. The changes in methoxyl content for
the unbleached and bleached pulps is summarized in Figure 2.14
mmol methoxy protons/gr lignin
16
14
12
10
8
6
4
2
0
Brownstock
D
D(E+O)
LS29
LS19
LS15
CK33
CK21
CK15
Figure 2.14. Methoxyl proton content for residual lignin from brownstock, D, and D(EO)
CK and LS pulps as determined by 1H NMR.
The decrease in methoxyl content from the brownstock to the Rapid D0 bleached pulps is a result
of the demethoxylation chemistry associated with chlorine species in the D0 stage. The
additional decrease in methoxyl content measured after the (EO) stage can be attributed, in part,
to the hydrolysis of muconic acid methyl esters during alkaline extraction. Nonetheless, the
largest loss in methoxyl content occurred during the Rapid D0 stage.
Another group of significant interest in chlorine dioxide bleaching is the phenolic unit. The use
of proton NMR provides a method of monitoring the changes in the concentration of this
functional group as pulping and bleaching proceeds. The results of this analysis for the LS and
CK pulps are summarized in Figure 2.15.
mmol phenolics/gr lignin
3
Brownstock
D
2.5
D(E+O)
2
1.5
1
0.5
0
LS29
LS19
LS15
CK33
CK21
CK15
Pulps
Figure 2.15. Phenoxy proton content for residual lignin from brownstock, D, and D(EO)
CK and LS pulps as determined by 1H NMR.
The results of this analysis suggested that the Rapid D0 stage was efficient at removing most of
the phenolics from the residual lignin isolated for these studies.
mmol acid units/gr lignin
2
Brownstock
D
D(E+O)
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
LS29
LS19
LS16
CK33
CK21
CK15
Pulp
Figure 2.16. Carboxylic acid proton content for residual lignin from brownstock, D, and
D(EO) CK and LS pulps as determined by 1H NMR.
As expected, the extensive oxidation of lignin yielded a residual lignin that contained a
significant enrichment of acid groups, as summarized in Figure 2.16.
In light of these substantial structural changes in the residual lignin, the parameters contributing
to differences in bleachability become more difficult to elucidate. Nonetheless, a close
inspection of the residual lignin components of the starting pulps did suggest a possible
contributing factor. The CK pulps studied in this report have both higher noncondensed and
condensed phenoxy structures. Perhaps most notable is the increased amount of condensed
phenolics.
1.8
mmol condensed guaiacyl/gr lignin
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
LS29
LS19
LS16
CK32
CK21
CK15
Pulp
Figure 2.17. Condensed guaiacyl proton content for residual lignin for CK and LS pulps as
determined by 31P NMR.
The results of the 31P NMR analysis of the phosphitylated residual lignin samples suggests that
the conventional pulps have higher amounts of condensed lignin than the EK pulps. Hence, the
CK pulps, despite having higher total phenolic concentration, are less reactive to chlorine
dioxide due possibly to the increased amounts of condensed phenolics.
It is interesting to compare the results of this analysis with our prior studies on conventional and
EMCC® pulps. The results of pulp bleachability studies for these latter pulps and the phenolic
content of the residual lignins are summarized in Table 2.4.
Table 2.4. Kraft residual lignin structure and bleachability of EMCC® (E) and
conventional softwood kraft pulps bleached via D0(E+O).
Pulp1
TAC2/∆
Kappa
mmol condensed
mmol total
phenolics/gr phenolics/gr
lignin
lignin3
E:29
0.21
2.07
0.91
E:18
0.22
2.15
0.94
C:28
0.21
2.07
0.89
C:18
0.23
2.29
0.99
1
E pulps were prepared via simulated EMCC® conditions, C pulps were prepared under simulated conventional
kraft conditions; kappa number of the unbleached softwood kraft pulp is given after the letter; 2pulps were bleached
under conventional D0 (0.20 k.f.) (E+O) stages (see Froass et al.25 for experimental further details); 3lignin isolated
in the same manner as employed in this paper.
Our results with the CK pulps mirror the previous published results summarized in Table 2.4.
As the kappa number of our CK or C pulp was lowered from the 30s to the mid-20s the phenoxy
content increased and bleachability decreased. In each case, this can be attributed to the increase
in condensed phenolic lignin units. The LS pulps do not have the same profile of condensed
lignin, and presumably this explains the differences in bleachability between the LS pulps and
the E pulps in Table 2.4.
Along with the residual lignin samples selected, bleach effluents were isolated and characterized.
The combined effluents from the CK33 pulp bleached D(EO) were collected along with the
effluents from replacing (EO) with E, EOP. Replacing the sequence D(EO) by DE decreased
delignification efficiency by 7%, whereas the addition of 0.5% P in the (EOP) stage improved
delignification by 6%. The residual lignin from the D(EO), DE, D(EOP) bleached pulps and
effluents were collected and analyzed by 31P NMR. The results of this analysis are summarized
below in Figure 2.18 and these measurements must be viewed in a semi-quantitative manner
since the recovery of this highly oxidized lignin was typically less than 25%.
Carboxyl groups, mmol/g lignin .
1.00
0.90
Residual
Effluent
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
BS
D
D(E)
D(E+O)
D(E+O+P)
Bleaching Stage
Figure 2.18. Carboxyl groups of residual lignin from CK33 brownstock and after D0, DE,
D(EO), and D(EOP) treatments along with effluents.
2.4 EXPERIMENTAL PROCEDURES:
Materials
All reagents and solvents were commercially purchased and used as received, except for the pdioxane, which was distilled in the presence of NaBH4. NMR solvents were purchased
anhydrous and stored over 4Å molecular sieves. Deionized water was used for all operations
except the lignin isolation procedure. Filtered nanopure deionized water was used for the lignin
isolation procedure. Chlorine dioxide was generated from the reduction of sodium chlorate with
oxalic acid employing 9 N sulfuric acid. The small amount of chlorine that was generated by
this process was measured and converted to chlorine dioxide by the stoichiometric addition of
sodium chlorite.
A single mature loblolly pine tree grown in the southeastern United States was employed for all
pulping studies reported in this paper. The tree was debarked, chipped, and carefully screened.
Figures 2.19 and 2.20 summarize the physical properties of the accept wood chips.
45.0
40.0
35.0
Fraction, %
30.0
25.0
20.0
15.0
10.0
5.0
0.0
0-2
2-4
4-6
6-8
Chip Thickness, mm
8-10
10>
Figure 2.19. Average chip thickness, mm.
60.0
Fraction, %
50.0
40.0
30.0
20.0
10.0
0.0
0-3
3-7
7-12.7 12.7-25.4 25.4-45
Chip Size, mm
45>
Figure 2.20. Average chip size distribution.
SW Kraft Pulping
The kraft pulping experiments were performed at Ahlstrom Machinery Corp. in Glens Falls, NY.
Established procedures17 were used to simulate conventional and Lo-Solids® kraft cooking
procedures. Table 2.5 summarizes the cooking parameters employed for the SW kraft cooks and
the physical properties of the pulps.
Conventional and Lo-Solids® SW kraft pulping conditions and pulp properties from
the CK and LS pulps.
Table 2.5
Lo-Solids® Kraft Pulps (LS)
Conventional Kraft Pulps (CK)
Pulp
CK33 CK28 CK21 CK20 CK18 CK15 CK13 LS29
LS26
LS19 LS17 LS16 LS11
Kappa #
33.0
28.3
21.3
20.4
18.1
14.7
13.3
29.3
26.1
19.1
17.1
16.0
11.0
Total Yield 48.6
% on Wood
48.1
46.6
47.4
46.8
44.7
44.0
48.3
47.8
45.6
44.9
44.2
41.8
Max. Temp 168
170
170
170
171
171
171
160
163
166
169
170
170
1201
1213
1999
2311
2987
3496
3499
2003
2442
3362
4126
4489
4474
14.8
14.0
15.4
14.9
15.3
16.7
16.4
14.1
15.2
15.0
15.9
16.6
17.8
H-Factor
Total EA
Consumed
% on
wood
NaOH
Residual Lignin Isolation and Characterization
Residual lignin was isolated from the various pulps by employing an aqueous acidic-dioxane
solution following literature procedures.22 In brief, the procedure required an air-dried SW kraft
(50-150 gr o.d.) to be placed in a flask with a solution of 0.10 M HCl 9:1 dioxane:water at 8%
consistency. The pulp slurry was refluxed for 2 h under argon before filtering through celite and
a medium glass-fritted funnel. The celite was washed with an additional 500 mL of p-dioxane.
The filtrate was neutralized with a saturated sodium bicarbonate solution before the p-dioxane
was removed under reduced pressure at 40°C. The lignin was precipitated by acidification with
1.00 N HCl. The lignin was further coalesced by freezing, thawing, and centrifuging. The
supernatant was decanted to the level of the precipitated lignin. This washing procedure was
performed 3 times before the lignin was freeze dried. On average, the yield of residual lignin
from the CK and LK kraft was 55% (see Table 2.6).
Table 2.6. Yield of lignin recovered from lignin isolated from SW kraft brownstock pulps.
Conventional
Kraft Pulp
% Lignin
Recovereda
Lo-Solids®
Kraft Pulp
% Lignin
Recovered
CK33
58
LS29
52
CK28
52
LS26
49
CK21
54
LS19
48
CK20
55
LS17
53
CK18
51
LS16
55
CK15
57
LS11
45
CK13
66
-
-
a
% lignin recovered=(mass of lignin recovered)/(kappa number of pulp x
oven-dry weight of pulp x 0.147%).
Table 2.7. Yield of lignin recovered from bleached SW kraft pulps.
CK Pulp
%Lignin
EK Pulp
%Lignin
Yielda
Yield
EK29.3:
CK32.4:
D0
35
30
D0
36
36
(EO)
(EO)
CK21.3:
EK19.1:
D0
29
D0
30
(EO) 29
(EO)
28
EK15.6:
CK14.7:
D0
30
28
D0
29
(EO) 39
(EO)
a
% lignin recovered=(mass of lignin recovered)/(kappa number of pulp x oven-dry weight of
pulp x 0.15%)
Quantitative 1H NMR analysis of the residual lignin samples was accomplished employing
underivatized lignins in DMSO-d6 using trimethyl silyl propionate-d4 (TSP) as the internal
standard. 1H NMR measurements were accomplished following the method of Li and
Lundquist39 employing a Bruker 400 MHz DMX spectrometer. Operating conditions included a
90-degree pulse, a TD of 32 k, a 15 sec. delay, and a sweep width of 24 ppm. The NMR
experiments were performed at 50°C on samples containing 35-50 mg lignin/mL of DMSO-d6.
The Fourier transformed spectra were integrated according to reported chemical shifts for lignin
functional groups relative to an internal standard sodium-3-trimethylsilyl-propionate-2,2,3,3-d4
(TSP). Using the known weight of the TSP in solution, the lignin functional group integrals
were transformed into units of mmol/g isolated lignin.
Hydroxyl functional groups in isolated lignins were quantified by 31P NMR spectroscopy using a
slight modification of the procedure developed by Argyropoulos.40 Approximately 35 mg of
dried isolated lignin was weighed into a 2 mL vial. To the vial was added 280 µL pyridine
containing chromium acetylacetonate (2 mg/mL) and cyclohexanol (2 mg/mL). After the lignin
was completely solubilized by the pyridine solvent, 175 µL CDCl3 was added with stirring.
Fifteen minutes before NMR analysis, 75 µL 2-chloro-4,4,5,5-tetramethyl-1,3,2–
dioxaphospholane was added to the lignin solution with vigorous stirring.
Quantitative 31P NMR analysis was acquired under the following conditions: 25–second pulse
delay, inverse-gated decoupling (Waltz-16), 30° pulse angle, a time domain of 32K with one
degree of zero filling, 200 acquisition transients, and 4.0 Hz line broadening. Signal assignments
and integration regions were based on literature methods.40
BLEACHING STUDIES:
Rapid D0
Employing the quantum reactor, a 3% consistency pulp mixture was heated to 45oC and the pH
was adjusted to either 5.0 or 4.0 for kappa factor 0.20 and 0.05 ClO2 bleaching experiments,
respectively. ClO2 (kappa factor of either 0.05 or 0.20) was added into the quantum reactor and
the pulp was mixed for 30 sec at 15 Hz, the mixer was then stopped for 30 sec, after which a
quenching solution of aqueous Na2SO3 (mass Na2SO3 added=mass applied ClO2 x 4.67 ) was
injected. The pulp mixture was then stirred for an additional 30 sec., filtered, and characterized.
Kappa number was then determined.
(EO)-Stage
The pulp from the D0 stage was thickened to 25% consistency, without washing, thoroughly
disintegrated, and added to a pin mixer. The consistency was adjusted to 10%, and the pulp was
then extracted under typical (EO) conditions: NaOH charge was chosen to yield a terminal pH of
10.7-11.5. The (EO) stage was performed at 70oC for 60 min. The initial O2 pressure was 60
psig, and this was decreased 10 psig every 5 min. At the end of the (EO) stage, the pulp
consistency was adjusted to 3%, the effluents were collected, and the pulp was thoroughly
washed.
CHAPTER 3: STUDIES OF CHLORINE DIOXIDE DELIGNIFICATION: VAPOR PHASE
BLEACHING OF HARDWOOD AND SOFTWOOD KRAFT PULPS
3.1 INTRODUCTION:
“Vapor phase” delignification of unbleached kraft pulp consists of contacting it, at high
consistency, with a gas stream containing ClO2. This mode of ClO2 application has been
proposed as a means of achieving efficient delignification while simultaneously generating
relatively small amounts of byproduct adsorbable organic halide (AOX).41,42 Our interest in this
process has been as part of a study of the interdependence of delignification efficiency,
bleaching conditions, and residual lignin structural changes. Accordingly, we have conducted
vapor phase ClO2 delignification experiments in the laboratory and have followed them with
determinations of residual lignin structural changes and effluent AOX. The emphasis of this
study was on hardwood kraft pulps made from sweetgum by laboratory simulations of both
conventional and RDH kraft pulping. As a comparison, some experiments were also conducted
on pulps made from southern pine by laboratory simulations of conventional kraft pulping.
3.2 RESULTS AND DISCUSSION:
Results indicative of D0(EO) delignification efficiency are contained in Table 3.1 for the
hardwood pulps and in Table 3.2 for the softwood pulps. As shown in Table 3.1, vapor phase
delignification of the hardwood pulps was remarkably more efficient than conventional, low
consistency bleaching. In spite of the fact that less ClO2 was consumed in the vapor phase
experiments, the kappa number after extraction was 50% lower. This is in agreement with
Mendiratta’s1 earlier observation that delignification at high consistency with a gaseous mixture
of ClO2, O2, and water vapor gave much better delignification efficiency than conventional
medium consistency delignification. In the present case, hardwood pulp delignification
efficiency in the D0(EO) partial sequence, expressed as kappa number reduction per percent of
active chlorine consumed, was 4.4-5.3 for the vapor phase D0 stage, as compared with 3.0-3.9 for
the low consistency D0 stage.
In the case of the softwood pulps, as shown in Table 3.2, there was a much smaller improvement
in delignification efficiency in going from low consistency to vapor phase bleaching. This
observation differs from those made earlier by Mendiratta1 and Reeve43, indicating that vapor
phase delignification efficiency is sensitive to operating conditions and perhaps also to
unbleached pulp characteristics. In this regard, it is worth noting that we made no attempt to
optimize the vapor phase ClO2 stage, either for bleaching performance or effluent quality.
Several important observations not directly related to the comparison between vapor phase and
conventional bleaching may also be made with regard to the data in Tables 3.1 and 3.2. One
confirms the observation, made previously in this laboratory and elsewhere, that delignification
efficiency exhibits a modest decrease as unbleached kappa number is decreased. The data of
Table 3.2 also reveal a very large effect of kappa factor on delignification efficiency, the
efficiency being more than twice as high at kappa factor 0.05 than at 0.2.
Table 3.1. Hardwood Pulp Bleaching Data.
Unbl.Pulp
Experiment
Process
HCC1
HCC2
HCR1
HCR2
HCR3
Conventional
HVC1
HVC2
HVR1
HVR2
HVR3
Vapor Phase
Pulp
Type
Conv.
RDH
Conv.
RDH
D0 Stage
Kappa
Number
11.4
15.2
7.8
13.4
22.9
Kappa
Factor
0.200
0.200
0.200
0.200
0.200
Kappa
Number
6.2
7.9
5.6
10.0
13.9
11.4
15.2
7.8
13.4
22.9
0.181
0.171
0.155
0.172
0.167
4.2
5.3
3.3
5.4
8.2
(EO) Stage
Delta NaOH
Kappa/ Charge,
TAC % o.d.p.
2.3
2.5
2.4
2.0
1.4
1.7
1.3
1.7
2.0
1.7
3.5
3.8
3.7
3.5
3.8
1.7
1.7
1.7
1.7
1.7
Final
pH
12.4
12.0
11.9
12.0
11.6
11.8
11.8
11.9
11.8
10.6
Delta
Kappa Kappa/
Number TAC
4.2
3.2
5.2
3.3
3.1
3.0
5.2
3.1
5.2
3.9
2.3
2.5
1.5
2.8
2.6
4.4
4.9
5.2
4.6
5.3
Notes: 1. D0 stage conditions: For conventional bleaching, Quantum mixer, 30 min., 45oC, 3% consistency, final pH 2.5-3.0. For vapor phase bleaching, rotating spherical glass reactor, 7-10
min., 60oC, 30% consistency, final pH 2.5-3.0.
2. (EO) stage conditions: Pin mixer, 60 min., 70oC, 10% consy., oxygen pressure initially 60
psig. and decreased by 10 psig. every 5 min.
Finally, it is apparent that there is little difference in D0(EO) delignification efficiency between
hardwood and softwood pulps, or between conventional and RDH pulps, at least over the ranges
of kappa numbers investigated in this study. This suggests that the relative ease of bleaching of
hardwood pulps has more to do with their lower unbleached kappa numbers than with any
inherent difference in ease of lignin removal.
Information on the mechanisms of delignification and potential environmental effects was
obtained by collecting and analyzing filtrates from each of the bleaching stages. The resulting
data are presented in Tables 3.3 and 3.4. The yield figures shown in these tables represent the
Table 3.2. Softwood Pulp Bleaching Data.
Unbl. Pulp
Experiment
Process
SCC1
SCC2
SCC3
SCC4
Conventional
SVC1
SVC2
Vapor Phase
Pulp
Type
Conv.
Kappa
Number
18.7
30.4
33.0
30.7
(EO) Stage
D0 Stage
Kappa
Factor
0.050
0.200
0.200
0.200
0.050
0.184
Delta NaOH
Kappa Kappa/ Charge,
Number TAC % o.d.p.
14.2
4.8
1.2
8.8
2.6
1.8
14.6
2.6
2.5
12.0
3.2
2.6
26.4
15.4
2.8
2.7
2.5
2.5
Final
pH
11.2
11.6
11.2
11.2
12.2
11.8
Delta
Kappa Kappa/
Number TAC
10.2
9.1
3.9
4.0
4.2
4.3
4.6
4.3
15.8
4.5
9.7
4.6
Notes: 1. D0 stage conditions: For SCC1 and SCC2, Quantum mixer, 30 min., 45oC, 3% consistency,
final pH 2.3-2.8. For SCC3 and SCC4, Quantum mixer, 45 min., 45oC, 10% consistency, final
pH 2.0. For SVC1 and SVC2, rotating spherical glass reactor, 8-10 min., 60oC, 30% consistency, final pH 2.6-2.8.
2. (EO) stage conditions: Pin mixer, 60 min., 70oC, 10% consistency, oxygen pressure initially 60
psig. and decreased by 10 psig. every 5 min.
percentages of the chlorine atoms in the ClO2 charged that are converted to the indicated species.
Striking features of the hardwood data (Table 3.3) are the markedly higher rate of AOX
generation in the vapor phase process than in the conventional one, the high levels of AOX in
even the conventional process, and the decreased AOX yield when low kappa number pulps are
bleached. These effects are also graphically shown in Figure 3.1. Vapor phase delignification
generated AOX at more than twice the rate of the conventional process, which in turn generated
more than expected on the basis of a model based on a review of the literature on softwood
bleaching with mixtures of ClO2 and Cl2.44 The latter study showed that delignification of
softwoods in the (DC)E partial sequence with mixtures of ClO2 and Cl2 rich in ClO2 converted
about 8% of the charged Cl atoms into AOX. The corresponding figures for D0(EO)
delignification of hardwood pulps from the present study (averages from Table 3.3) are 14% for
a low consistency D0 stage and 33% for a vapor phase D0 stage. Tentative conclusions are that
hardwood pulps give a higher yield of AOX than softwoods, and that vapor phase delignification
of hardwood pulps generates abnormally large quantities of AOX under the conditions employed
in this study. The higher AOX levels in vapor phase delignification may be indicative of gas
phase decomposition of ClO2 prior to its reaction with the lignin in the pulp, which would also
explain the higher delignification efficiency because of the known superiority of Cl2 - ClO2
mixtures in this respect. The composition of the mixture of inorganic byproducts is consistent
with this explanation, since the vapor phase filtrates contained somewhat lower concentrations of
chloride ion, and a greater proportion of the chloride ion produced in the vapor phase process
was released in the alkaline extraction stage. Both of these changes are to be expected when
substitution assumes a greater role relative to oxidation. The vapor phase filtrates also contained
markedly less chlorate ion (Figure 3.2), as would be expected if ClO2 had decomposed to Cl2
before reacting. Regardless of the mechanism, the lower rate of chlorate production is consistent
with the higher delignification efficiency of the vapor phase process.
20
D0 + (EO) Chlorate Yield, %
D0 + (EO) AOX Yield, %
40
35
30
25
20
15
10
Conv. CK
Vapor CK
5
Conv. RDH
Vapor RDH
0
15
10
5
Conv. CK
Vapor CK
Conv. RDH
Vapor RDH
0
5
10
15
20
Unbleached Kappa Number
25
Figure 3.1. Sum of AOX yields from D0
analyses of D0 and (EO) stage filtrates.
5
10
15
20
Unbleached Kappa Number
25
Figure 3.2. Sum of chlorate yields from analysis
and (EO) stage filtrates.
Qualitatively similar observations may be made with regard to the softwood pulp data presented
in Table 3.4, but the trends were generally less pronounced than in the case of the hardwood
pulps. Thus, the disparity between the rates of AOX generation in the vapor phase and
conventional processes, though still great (18% vs. 10%), was smaller than in the case of
hardwoods (33% vs. 14%). The AOX levels for the conventional process were only slightly
greater than predicted by the model referred to earlier, and the effect of kappa number on AOX
yield was either absent or too small to be apparent. The reduction in chlorate yield associated
with going from the conventional process to the vapor phase process, unlike the trends just
mentioned, was just as pronounced for the softwood pulps as for the hardwoods. In addition,
positive effects of increasing kappa factor on the yields of both AOX and chlorate ion were
observed. Figures 3.3 and 3.4 illustrate these effects.
To elucidate the mechanisms of the conventional and vapor phase ClO2 delignification
processes, residual lignin was isolated from the conventional kappa no. 15.2 hardwood pulp
before bleaching, after the D0 stage, and after the (EO) stage, for both the conventional and
vapor phase bleaching processes. The residual lignin was isolated using an acidic dioxane
extraction procedure that has been employed by several researchers.45 The resulting lignin was
then purified, derivatized with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane, and
characterized by 31P NMR.47
25
D0 + (EO) Chlorate Yield, %
D0 + (EO) AOX Yield, %
20
15
10
5
Conv.
Vapor
15
10
5
Conv.
Vapor
0
0
0.00
20
0.05
0.10
0.15
Kappa Factor
0.20
0.25
Figure 3.3. Sum of AOX yields from
analyses of D0 and (EO) stage filtrates
from bleaching of softwood pulps.
0.00
0.05
0.10
0.15
Kappa Factor
0.20
0.25
Figure 3.4. Sum of chlorate yields from
analyses of D0 and (EO) stage filtrates from
bleaching of softwood pulps.
1.8
HW Kraft
Do Conventional
Do Vapour Phase
Do-conv.(EO)
Do-vapor phase(EO)
1.6
mmol functional group/gr lignin
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Aliphatic
Hydroxy
Groups
Phenolic
Hydroxy
Groups
Carboxylic
Acid
Figure 3.5. Changes in aliphatic, carboxylic, and phenolic hydroxy content for conventional
hardwood kraft pulp (kappa no. 15.2) bleached D0, 0.20 k.f., with vapor phase and
conventional conditions followed by an (EO) stage.
The results of this analysis are illustrated in Figure 3.5. As expected, the residual lignin was
found to be enriched in acid groups as the brownstock pulp was treated with ClO2 and
subsequently alkaline extracted in an (EO) stage. Interestingly, the residual lignin isolated from
the vapor phase D0 and D0(EO) pulps exhibits a slight enrichment in acid groups over the
conventional D0 bleached pulp. A slight decrease in aliphatic hydroxy group content after the D0
stage could be potentially attributed to some oxidation of the side chain of lignin. Both the
conventional and vapor phase D0 stages removed extensive amounts of phenolic groups from the
residual lignin, which was expected since the phenolic group is the primary site of oxidation by
chlorine dioxide.
The NMR methods employed in this study can further differentiate the phenolic groups into (a)
syringyl, (b) condensed, and (c) guaiacyl and demethylated phenolics. Figure 3.6 summarizes
the results of this analysis for the lignins studied in this report. Although all three types of
phenolic groups are removed during the D0 bleaching stage, it is clear that the syringyl units are
the most reactive toward chlorine dioxide. Interestingly, the loss of guaiacyl and condensed
types of phenolic groups appears to occur in comparable amounts. This result differs from our
previous
0.7
HW Kraft
Do Conventional
Do Vapour Phase
Do-conventional(EO)
Do-vapor phase(EO)
mmol functional group/gr lignin
0.6
0.5
0.4
0.3
0.2
0.1
0
Syringyl
Guaiacyl
+Demethylated
Condensed
Figure 3.6. Changes in syringyl, condensed, and guaiacyl/demethylated phenolic
hydroxy units for conventional hardwood kraft pulp (kappa no. 15.2) bleached D0,
0.20 k.f., vapor phase and conventional, followed by an (EO) stage.
studies in which we noted condensed phenolic groups to be resistant to oxidative bleaching
agents.46 The factors contributing to this trend will need to be further investigated so as to fully
determine the mechanism of oxidative removal of condensed phenolics.
Table 3.3. Fate of Chlorine in Hardwood Pulp Bleaching.
Unbl. Pulp
Expt.
Process
Pulp
Type
Kappa
Number
HCC1
HCC2
HCR1
HCR2
HCR3
Conv.
Conv.
HVC1
HVC2
HVR1
HVR2
HVR3
Vapor
RDH
Conv.
RDH
11.4
15.2
7.8
13.4
22.9
Kappa
Factor
0.200
0.200
0.200
0.200
0.200
AOX,
kg/t
0.541
0.834
0.294
0.737
0.916
11.4
15.2
7.8
13.4
22.9
0.181
0.171
0.155
0.172
0.167
1.180
1.639
0.705
1.400
1.445
D0 Stage
Yields of Cl Cpds, Mole % ClO2
Total
Yields of Cl Cpds, Mole % ClO2
(EO) Stage
Yields of Cl Cpds, Mole % ClO2
AOX
Cl-
ClO3-
ClO2-
AOX,
kg/t
AOX
Cl-
ClO3-
ClO2-
AOX,
kg/t
AOX
Cl-
ClO3-
ClO2-
Total Cl
Recovery, %
11.9
13.7
9.4
13.8
10.0
44
47
56
51
53
16
17
18
18
17
1
1
1
1
0
0.037
0.136
0.052
0.142
0.521
0.8
2.2
1.7
2.7
5.7
8
8
8
8
7
1
1
1
1
1
4
2
m.i.
m.i.
m.i.
0.571
0.960
0.261
0.589
1.152
12.7
16.0
11.1
16.4
15.7
52
55
64
59
60
18
18
19
19
19
5
3
m.i.
m.i.
m.i.
87
92
94
94
94
28.5
31.6
29.3
30.3
18.9
43
39
(67)
41
40
6
7
(11)
6
6
0
0
0
0
0
0.092
0.247
0.067
0.228
0.829
2.2
4.8
2.8
4.9
10.8
13
12
19
13
11
0
0
1
0
0
m.i.
m.i.
m.i.
m.i.
m.i.
1.259
1.867
0.765
1.612
2.251
30.8
36.3
32.1
35.3
29.7
56
51
(86)
53
51
6
7
(12)
6
6
m.i.
m.i.
m.i.
m.i.
m.i.
93
94
(130)
95
87
Notes: 1. m.i. - Value could not be determined because of matrix interference.
2. Analytes present at concentrations below their detection limits were assumed to be present at a concentration of one-half of the detection limit.
Table 3.4. Fate of Chlorine in Softwood Pulp Bleaching.
Unbl. Pulp
D0 Stage
Yields of Cl Cpds, Mole % ClO2
(EO) Stage
Yields of Cl Cpds, Mole % ClO2
Total
Yields of Cl Cpds, Mole % ClO2
Expt.
Process
Pulp
Type
Kappa
Number
Kappa
Factor
AOX,
kg/t
AOX
Cl-
ClO3-
ClO2-
AOX,
kg/t
AOX
Cl-
ClO3-
ClO2-
AOX,
kg/t
AOX
Cl-
ClO3-
ClO2-
Total Cl
Recovery, %
SCC1
SCC2
SCC3
SCC4
Conv.
Conv.
18.7
0.050
0.200
0.200
0.200
0.095
0.656
1.300
1.210
5.1
8.8
10.7
9.2
(114)
74
n.d.
n.d.
4
18
n.d.
n.d.
2
1
n.d.
n.d.
0.068
0.176
0.130
0.160
3.6
2.4
1.1
1.2
(124)
15
n.d.
n.d.
12
3
n.d.
n.d.
2
1
n,d,
n.d.
0.176
0.772
1.43
1.37
8.7
11.1
11.8
10.4
(238)
89
n.d
n.d.
16
21
n.d
n.d.
4
2
n.d.
n.d.
(268)
123
n.d.
n.d.
SVC1
SVC2
Vapor
0.050
0.184
0.297
1.261
9.6
11.2
75
56
2
10
1
0
0.216
0.846
7.0
7.5
19
15
0
0
1
0
0.476
1.955
16.6
18.6
94
71
2
10
2
0
115
100
30.4
33
30.7
Notes: 1. n.d. - Not determined.
2. Analytes present at concentrations below their detection limits were assumed to be present at a concentration of one-half of the detection limit.
3.3 EXPERIMENTAL:
HW Kraft Pulp Bleaching
Vapor phase D0 stage bleaching was done by passing a stream of nitrogen and chlorine dioxide
through a rotating spherical flask containing fluffed pulp at 30% consistency maintained at 60oC.
The gas stream was generated by stripping ClO2 from a 6.2 g/L solution containing a negligibly
small amount of Cl2. The flask containing ClO2 solution, initially at 4oC, was placed in a 90oC
water bath, and the flow of N2 was immediately started. An amount of ClO2 solution containing
3 g ClO2 was used in every case, and the amount of ClO2 delivered to the reaction flask was
controlled by controlling the N2 flow rate and the duration of the experiment. For example, 2.92
g ClO2 was delivered by passing N2 through the solution for 10 min. at a flow rate of 6.5 L/min.
The gas leaving the reactor was passed through 10% KI, which was subsequently titrated
iodometrically to determine the amount of unreacted ClO2. HW pulps supplied with an amount
of ClO2 corresponding to a kappa factor of 0.20 consumed 77-91% of the applied ClO2,
depending on the unbleached kappa number. Kappa no. 30.7 SW kraft pulp, when supplied with
an amount of ClO2 corresponding to a kappa factor of 0.276, consumed 67%, an amount
corresponding to a kappa factor of 0.184. For all pulps, the ClO2 was completely consumed
when the amount supplied corresponded to a kappa factor of 0.05.
Conventional D0 stage bleaching was done in a Quantum reactor at 3% consistency and 45oC for
30 min. The mixer was turned on during ClO2 addition, for 15 sec. thereafter, and for 5 seconds
every 5 min. throughout the remainder of the bleaching period. Both types of D0 stage were
followed by an (EO) stage conducted in a laboratory pin mixer at 10% consistency and 70oC for
60 min. The oxygen pressure was initially 60 psig. and was decreased by 10 psig. every 5 min.
Filtrate samples were collected immediately upon termination of each bleaching stage. After
vapor phase D0 stages and (EO) stages, the pulp was diluted to 3% consistency and thoroughly
mixed before filtering. Samples for AOX determination were acidified to pH 2 with concentrated
HNO3. All samples were refrigerated pending analysis. Determinations of AOX were made
according to EPA method 1650. Chloride, chlorate, and chlorite were determined by capillary
ion electrophoresis.
The unbleached pulps were prepared in the laboratory by standard methods. All of the HW kraft
pulps were prepared from fresh sweetgum chips at the laboratories of Beloit Corp. A SW pulp of
18.7 kappa number was prepared at IPST. The remaining softwood pulps were provided through
the courtesy of J. Jiang and K. Crofut of Ahlstrom Machinery Inc., who prepared them from
southern pine chips supplied by IPST (see Chapter 2).
Lignin isolation
The isolation of lignin from the kraft pulps was accomplished employing standard literature
methods.5 In brief, air-dried pulp (60 g oven-dry weight) was added to an aqueous 0.1 N HCl
(146 ml 1 N HCl), p-dioxane (1300 ml freshly distilled) solution, and this mixture was then
refluxed for 2 hr. under an argon atmosphere. The mixture was then filtered, and the filtrate was
filtered through celite. The solution was neutralized with sodium bicarbonate and concentrated
under reduced pressure. After the filtrate was vacuum distilled to less than 10% of the original
volume, water was added (3 x 200 mL) and the mixture was reconcentrated under reduced
pressure. The aqueous solution was then acidified pH to 2.5 with an aqueous 1 N HCl solution.
The resulting precipitate was collected, washed several times with distilled water, and freezedried.
Lignin NMR studies.
NMR data were acquired with a DMX 400 MHz Bruker spectrometer. Quantitative 31P-NMR
experiments were performed following standard literature methods.47
3.4 CONCLUSIONS:
Vapor phase ClO2 delignification of hardwood kraft pulps made from sweetgum is remarkably
more efficient than conventional low consistency ClO2 delignification. At 0.2 kappa factor,
delignification efficiency in the D0(EO) partial sequence, expressed as kappa number reduction
per percent of active chlorine consumed, was 4.4-5.3 for the vapor phase D0 stage, as compared
to 3.0-3.9 for the low consistency D0 stage.
In the case of southern pine kraft pulps, there was a much smaller improvement in delignification
efficiency in going from low consistency to vapor phase bleaching. This observation differs from
those made earlier by others, indicating that vapor phase delignification efficiency is sensitive to
operating conditions.
Vapor phase delignification of hardwood kraft pulps generated abnormally large quantities of
AOX under the conditions employed in this study, and conventional ClO2 delignification of
hardwood pulps generated more AOX than expected on the basis of softwood bleaching data.
AOX yields in D0(EO) delignification of hardwood pulps from the present study are 14% for a
low consistency D0 stage and 33% for a vapor phase D0 stage. These figures may be compared
with the 8% to be expected on the basis of earlier softwood pulp bleaching data.
Vapor phase filtrates contained somewhat lower concentrations of chloride ion, and a greater
proportion of the chloride ion produced in the vapor phase process was released in the alkaline
extraction stage. The vapor phase filtrates also contained markedly less chlorate ion. These
observations, as well as the observations of improved delignification efficiency and higher AOX
yields, are consistent with the hypothesis that ClO2 partially decomposes in the vapor phase
before reacting with the pulp.
Qualitatively similar observations were made in the vapor phase D0(EO) delignification of
softwood kraft pulps, but the trends were generally less pronounced than in the case of the
hardwood pulps. Under the conditions used in this study, vapor phase delignification produced
an AOX yield of approximately 18%, as compared with approximately 10% in conventional low
consistency ClO2 delignification. Positive effects of increasing kappa factor on the yields of both
AOX and chlorate ion were also observed.
Differences in the structure of residual lignin after conventional and vapor phase bleaching were
slight. This may indicate that the superior bleaching efficiency of the vapor phase process is due
to the diminished importance of reactions that occur between active chlorine compounds and
dissolved organic byproducts. The absence of such reactions would leave no evidence in the
residual lignin structure. This explanation is also consistent with the elevated AOX levels
resulting from vapor phase bleaching if it is assumed that part of the dissolved organic material
that consumes active chlorine in the liquid phase is chlorine substituted.
CHAPTER 4: IMPROVING THE BLEACHABILITY OF HARDWOOD KRAFT PULPS
4.1 INTRODUCTION:
Pulp bleachability is an issue of critical fundamental and practical interest in the pulp and paper
industry. The ability to tailor a bleaching sequence to yield target brightness values with
minimal use of bleaching chemicals and/or new capital equipment has obvious potential for
savings in operating and capital costs. Our previous studies48 have shown that the bleachability
of SW kraft pulps is largely influenced by the bleaching chemistry of lignin. The factors
controlling bleachability of HW kraft pulps are more complicated since both carbohydrates and
lignin contribute to the apparent kappa number of HW kraft pulps. The interaction of these two
components in controlling pulp bleachability is poorly understood since no reported studies have
examined how both components simultaneously influence bleaching chemistry.
The formation of hexenuronic acids during pulping (see Figure 4.1) is currently a very active
research area that has been shown to have practical mill considerations. Early studies by
Clayton49 examined the behavior of 4-O-methyl-α-D-glucuronoxylans from poplar, white birch,
and white elm with alkali at 170oC and found indirect evidence for the formation of hexenuronic
acids.1
4-O-methyl-D-glucuronoxylan
H
CO 2H
CH3O
O
HO
H
Hexenuronic Acid
CO2H
O
HO
NaOH
OH
OH
O
HO
+
O
O Xylan
HO
O
Figure 4.1
CH3OH
O Xylan
O
Alkali-catalyzed formation of hexenuronic acids.
Subsequent studies by Johansson and Samuelson50 and Simkovic et al.51 provided additional
experimental evidence supporting the formation of hexenuronic acids from 4-O-methyl-α-Dgluconoxylans. Finally, Teleman et al.52 isolated and characterized hexenuronic acids from an
enzymatically hydrolyzed unbleached kraft pulp.
Studies by Vuorinen et al.53 and Buchert et al.54 further explored the chemistry of hexenuronic
acids in kraft pulps. They demonstrated that hexenuronic acids contribute approximately 50% to
the kappa number of several northern Scandinavian kraft pulps. Furthermore, these unsaturated
sugars were shown to readily consume electrophilic bleaching chemicals such as chlorine
dioxide and ozone. A series of well-designed acid hydrolysis studies identified reaction
conditions under which hexenuronic acids could be removed from kraft pulps (see Figure 4.2)
without significantly impacting their physical properties.
OH
H
CH3O
HO
CO2H
O
O
Acid
OH
O
HO
+
O
O Xylan
OH
H
OH
OH
+
O
O
O
O
CO2H HOC
2-Furoic Acid
Figure 4.2.
O Xylan
HO
O
+ HCO 2H
CO2H
5-Carboxy-2-furaldehyde
Proposed acid hydrolysis pathway for hexenuronic acid.52
Following these reports, several investigators began to explore the impact of hexenuronic acids
on pulp bleaching operations. Li and Gellerstedt55 have examined the contribution of
hexenuronic acids to a pulp’s kappa number. Roberts56 and da Silva Filho et al.57 have reported
that the presence of hexenuronic acids is a principal factor controlling the affinity of pulp fibers
for nonprocess elements. Senior et al.58 reported that the use of AQ in a kraft cook could
influence the amount of hexenuronic acids present in HW kraft pulps. Nilvebrant and Reimann59
have shown that hexenuronic acids can contribute to oxalic acid formation from a Z-stage.
Lachenal and Chirat60 have demonstrated that the differing rates of reaction for chlorine dioxide
with lignin and hexenuronic acid can be employed to improve HW kraft bleaching operations.
Vuorinen et al.61 have studied the impact of hexenuronic acids in ECF and TCF bleaching
operations.
Despite this significant body of work, few reported studies had examined the presence of
hexenuronic acids from commercial North American kraft pulping operations prior to the studies
in this report. This phase of the research program studied several parameters:
•
Examined the concentration of hexenuronic acids in commercial SW and HW kraft pulps
from U.S. operations.
•
Evaluated how pulping parameters influence the formation of hexenuronic acids.
•
Examined the importance of the role of lignin and hexenuronic acids in controlling HW pulp
bleachability.
•
Developed low-cost hexenuronic acid removal technologies.
•
Evaluated full-sequence bleaching consequences of hexenuronic acids.
•
Evaluated the role of lignin structure in pulp bleachability of HW kraft pulps.
4.2 RESULTS AND DISCUSSION:
Evaluating the Role of Hexenuronic Acids in U.S. Kraft Pulps.
Although the presence of hexenuronic acids has been well documented for Northern European5355
kraft pulps, the presence of these unsaturated sugars in North American fiber sources has not
been as extensively studied. To examine this issue, we acquired pulp samples from several mills
in the U.S. Each pulp sample was exhaustively washed and analyzed for kappa number and
viscosity. The pulps were then refluxed at 3% consistency in a pH 3 buffered solution for 2 and
5 hours. Under these conditions, hexenuronic acids are released from the hemicellulose chains
and are converted to 5-carboxy-2-furaldehdye and 2-furoic acid. As shown in Figure 4.3, all the
commercial HW kraft pulps exhibited a significant reduction in kappa number after acid
hydrolysis ranging in value from 22% to 53%. Interestingly, the ease of acid-catalyzed removal
of hexenuronic acids from kraft pulp was found to vary substantially for some pulps. For most
of the HW kraft pulps examined, a two-hour acidic treatment significantly reduced the kappa
number of the pulp. For the pulp from Mill B, a five-hour acid stage was required to maximize
the drop in kappa number. The factors contributing to this diversity in response to a hot acid
stage are unknown.
Acid hydrolysis of commercial softwood kraft pulps was not as effective at reducing the kappa
number of the pulps, as shown in Figure 4.4. Repeating the acid stage with a series of
laboratory-prepared loblolly pine continuous extended modified kraft pulps yielded comparable
results as summarized in Figure 4.5. The results from acid hydrolysis of SW mill and lab kraft
cooks suggest that only a small proportion of the pulp kappa number can be attributed to acidsensitive hexenuronic acid groups. These results suggest that the minor presence of hexenuronic
acids is further reduced as delignification of SW kraft pulps is extended from kappa number 30
to a kappa number of 20.
K 15
a
p 10
p
a 5
#
0 h
2 h
5 h
0
Mill A
Mill B
Mill C
Mill D
Mill E Pre-O
Mill E Post-O
HW Pulp After (0,2,5 h) Acid Hydrolysis
Figure 4.3. Kappa number of commercial HW kraft pulps before and after refluxing in
pH 3 aqueous buffer solution for 2 and 5 h.
K
a
p
p
a
35
30
25
20
15
0 h
2 h
5 h
10
# 5
0
Conventional
Conventional
Extended
Pulp After (0,2,5 h) Acid Hydrolysis
Figure 4.4. Kappa number of commercial SW kraft pulps before and after refluxing in
an pH 3 aqueous buffer solution for 2 and 5 h.
25
0 h
2 h
5 h
20
K
a
p 15
p
a 10
#
5
0
SW Pulp After (0,2,5 h) Acid Hydrolysis
Figure 4.5. Kappa number of laboratory-prepared extended modified SW kraft pulps
before and after refluxing in aqueous formic acid-sodium formate solution for 2 and 5 h.
The acid hydrolysis stage not only lowers the kappa of the kraft pulps but also reduces the pulp
viscosity values. The HW kraft pulps typically incurred viscosity losses of 10 to 35% after
refluxing in the hot acid stage for 5 h. For the SW kraft pulps, the acid hydrolysis resulted in 2025% decreases in viscosity values. The acid hydrolysis conditions we employed were directed at
maximizing the removal of acid-sensitive functional groups from the pulps. Based on literature
considerations, the observed losses in viscosity could be minimized with additional optimization
studies.62
Along with the kappa number analysis of each kraft pulp, the xylan content of each starting pulp
was determined. Figure 4.6 presents the results of relating the xylan content of the kraft
brownstocks to the drop in kappa number after a 5-hour acid hydrolysis treatment. These results
suggest that there is a correlation between the xylan content of the starting pulp and the
contribution of hexenuronic acids to the kappa number of the pulp. This result is consistent with
the proposed mechanism of hexenuronic acid formation and its contribution to pulp kappa
number.
20
% xylose in pulp
18
16
14
12
10
8
6
4
2
0
Figure 4. 6.0 Relationship
xylan content
HW kraft pulps
10 between 20
30of SW and 40
50 and the
decrease of kappa number upon 5-hour acid hydrolysis stage.
% Kappa Decrease Upon Acid Treatment
In summary, the HW and SW studies reported in this section demonstrate that hexenuronic acids
are a significant contributor to the kappa number of HW kraft pulps, whereas they are only a
small component of the kappa number of SW kraft pulps. These results suggest that
approximately 20-50% of the bleaching costs associated with HW pulp bleaching operations are
due to hexenuronic acids and not lignin removal. The following section explores the relationship
between kraft pulping and the content of hexenuronic acids in HW kraft pulps and the
development of a low-cost hexenuronic acid removal stage.
Evaluating the Impact of Kraft pulping on Hexenuronic Acid Formation in HW Kraft
Pulps
To examine how kraft-cooking technologies can influence the formation of hexenuronic acids
for HW kraft pulps, we performed a series of conventional and extended modified batch cooks.
These cooks were all accomplished from a common sweetgum wood source to minimize
experimental variability due to the fiber source. Table 4.1 summarizes some of the important
cooking and pulp parameters for these HW kraft pulps.
Table 4.1. HW Kraft pulping conditions.
H-factor
%EA charged on wood
as Na2O
Kappa #
Total Yield
(%)
Conventional Kraft
1,800
15.7
11.4
48.8
1,400
14.9
11.8
49.5
1,100
13.6
14.4
50.3
800
12.8
15.2
51.0
RDH Kraft Pulp
1,000
NA
7.8
46.5
500
14.5
12.3
48.9
300
13.9
13.4
50.0
400
13.8
13.8
NA
250
NA
16.6
50.7
200
11.2
22.9
52.0
The conventional and RDH HW kraft pulps were refluxed in a pH 3 buffered solution for 5
hours, and the kappa number of the pulps was analyzed before and after the hot acid stage. The
results of the kappa number analysis are summarized in Figure 4.7. The data suggest that
differences in batch cooking do not significantly alter the formation of hexenuronic acids in kraft
pulps, although there does appear to be a small reduction in the formation of hexenuronic acid
groups for the RDH pulps. Of greater significance is the observed increase in kappa number that
can be attributed to hexenuronic acid groups as the final pulp kappa number is reduced. The acid
hydrolysis data presented in Figure 4.7 suggest that the HW kraft pulps with a kappa number of
approximately 14 have the largest amount of hexenuronic acids contributing to the observed
kappa number. As the delignification is extended, the pulps respond less favorably to an acid
hydrolysis stage. This is presumably due to a loss of acid-sensitive hexenuronic acid groups to
the kraft cooking conditions.
Along with the kappa number analysis of the acid hydrolyzed pulps, the hydrolysis effluents
were analyzed by UV/Vis for the presence of 2-furoic acid. As summarized in Figure 4.2, 2furoic acid can be used to measure the amounts of hexenuronic acids released from the pulp
during a hot acid stage. The results of this analysis for the acid pulps are summarized in Figure
4.8, and the data agree with the drop in kappa number observed for the acid hydrolyzed pulps. It
furthermore supports the hypothesis that for the sweetgum pulps studied, the content of
hexenuronic acid groups in pulp increases as delignification is extended from kappa number 30
to approximately 14. Further depletion of hexenuronic acids was observed as delignification was
extended. This is consistent with studies54 that reported that the hexenuronic acids undergo a
slow degradation during the latter part of a kraft cook.
% Delignification After Acid Hydrolysis
60
50
40
Conventional
30
RDH
20
10
0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
Kraft Pulp (Brownstock Kappa #)
Figure 4. 7. Change in kappa number for HW kraft pulps upon acid hydrolysis (5 hour
at 100oC, 3% csc, and buffer pH 3 solution) versus starting kappa number of conventional
and RDH kraft pulps.
60
HexA µmol/g pulp,
50
40
30
20
RDH
10
Conventional
0
0
10
20
30
40
Unbleached Kappa No.
Figure 4.8.
Hexenuronic acid content of unbleached RDH and conventional kraft pulps
from sweetgum as determined by measuring the concentration of 2-furoic acid in the
effluents from acid hydrolysis stages of the HW kraft pulps.
In summary, most U.S. HW kraft pulp bleaching operations utilize pulps that contain significant
amounts of hexenuronic acids. These sugars consume approximately 20-50% of bleaching
chemicals. The formation of hexenuronic acids cannot be readily precluded by changes in
conventional and/or RDH cooking technologies. In the following section, development of a
low-cost acid hydrolysis system is reported. The system was found to efficiently mitigate the
effects of hexenuronic acids in HW kraft pulps.
Development of a low-cost acid hydrolysis-stage for HW kraft pulps.
D0 Filtrate as Hydrolysis Medium
In their pioneering work on hexenuronic acid removal prior to bleaching, Vuorinen et al.53 used
buffers (mixtures of formic acid and sodium formate) as hydrolysis media. They also used
sulfuric acid, as did Siltala et al.63 in their mill-scale implementation of the process. Recycled
filtrate from the washer following a D0 stage may also have utility as a hydrolysis medium,
replacing at least part of the required acid. Table 4.2 compares the results of mild acid hydrolysis
of four sweetgum kraft pulps in D0 filtrate with the corresponding hydrolyses in pH 3 formate
buffer. It is apparent that the two media gave similar results. Kappa number reductions varied
from 16 to 52%, being higher for the pulps of lower kappa number. Apparent viscosity losses
ranged from 0 to 36%. There were no significant differences in behavior between conventional
and RDH pulps, nor between the two types of acid hydrolysis medium. These results
demonstrate the potential usefulness of recycled D0 stage filtrate for this application.
Effects of Hydrolysis Time and Temperature
The long retention times that have been used for complete removal of hexenuronic acid are
disadvantageous. They require very large retention vessels or makeshift use of high-density
storage towers, which commonly exhibit broad residence time distributions, leading to
incomplete hydrolysis and poor selectivity. An alternative is to employ shorter retention times
either by increasing the temperature or accepting a less-than-complete removal of hexenuronic
acid that is nevertheless sufficiently large to justify the implementation of the hydrolysis stage.
The data shown in Table 4.3 were obtained to address these possibilities. When the temperature
was maintained in the 90-95oC range, reducing the retention time from 5 h to 1 h caused a
roughly 50% reduction in the extent of hexenuronic acid removal. Reducing the time to 2 h and 3
h caused reductions in hexenuronic acid removal of 30% and 10%, respectively. It thus appears
that it is possible to achieve appreciable extents of removal at retention times in the 2-3 h range.
The use of a pressurized prehydrolysis vessel to allow the temperature to be raised to 100oC or
higher is another possibility. Table 4.3 shows that hydrolysis for 2 h at 100oC reduces the kappa
number to the same extent as 5 h at 95oC. Increasing the temperature to 125oC allows the
retention time to be shortened to 1 h with no loss in removal efficiency, or to fifteen minutes
with a 20% loss in efficiency. Viscosity losses were modest under all conditions.
T a b le 4 .2 . H y d r o ly s is o f S w e e t g u m K r a f t P u lp b y D 0 F ilt r a t e a n d F o r m a t e B u f f e r
K a p p a N o . R e d u c tio n
V is c o s ity A fte r
o n H y d ro ly s is
H y d ro ly s is , m P a .s
U n b le a c h e d P u lp
W ith
W ith
W ith D 0
W ith D 0
F o rm a te
F o rm a te
V is c o s ity ,
T ype
K appa N o.
m P a .s
F iltra te 1
F iltra te 1
B u ffe r2
B u ffe r2
C o n v e n tio n a l
2 9 .1
1 2 .9
6 0 .6
2 7 .4
6 .4
6 .2
4 .7
6 .1
4 0 .4
2 2 .7
3 8 .9
2 4 .1
RDH
1 7 .3
1 1 .8
5 1 .8
3 7 .8
5 .0
6 .1
5 .3
5 .7
3 5 .1
3 7 .8
3 9 .5
2 8 .7
1
P u lp a c id ifie d to p H 4 a t ~ 2 % c o n s is te n c y a n d th ic k e n e d to ~ 2 5 % c o n s is te n c y p rio r to
d ilu tio n w ith D 0 filtra te to 4 % c o n s is te n c y . H y d ro ly s is c o n d itio n s : 9 5 oC , 5 h . D 0 filtra te
g e n e r a te d b y b le a c h in g s w e e tg u m p u lp ( k a p p a n o . 1 1 .8 ) w ith C lO 2 ( k a p p a f a c to r 0 .2 0 )
3 0 m in a t 4 5 oC in a Q u a n tu m m ix e r (fin a l p H 2 .7 ).
2
P u lp in itia lly a t ~ 3 5 % c o n s is te n c y d ilu te d to 3 % c o n s is te n c y w ith 1 0 m M fo rm a te b u ffe
f o llo w e d b y a d ju s tm e n t to p H 3 .0 w ith H 2 S O 4 . H y d r o ly s is c o n d itio n s : 1 0 0 oC , 2 h .
T a b le 4 .3 . E f f e c ts o f H y d r o ly s is T im e a n d T e m p e r a tu r e
U n b le a c h e d P u lp
V is c o s ity
K appa N o.
A fte r
R e d u c tio n
H y d ro ly s is ,
on
m P a .s
H y d ro ly s is
T ype
K appa
N o.
RD H
1 6 .7
5 1 .8
4
F iltra te 1
90
1
2
5
2 .0
3 .2
4 .6
4 7 .7
4 4 .1
3 4 .6
C onv
1 2 .0
3 2 .9
12
F iltra te
2
95
1
3
5
3 .3
4 .7
5 .2
2 8 .8
2 6 .6
2 3 .3
RD H
1 7 .3
5 1 .8
4
F iltra te 3
95
5
5 .0
3 5 .1
F o rm a te 4
100
2
5 .3
3 9 .5
95
5
5 .6
2 4 .9
125
0 .2 5
1
4 .6
5 .9
2 9 .1
2 4 .4
C onv.
1
H y d ro ly s is C o n d itio n s
C o n s is T e m p . T im e ,
V is c o s ity , te n c y ,
m P a .s
%
M e d iu m
, oC
h
1 2 .0
3 2 .9
12
H 2S O
5
4
P u lp in itia lly a t ~ 3 5 % c o n s is te n c y d ilu te d to 4 % c o n s is te n c y w ith D
0
filtra te , fo llo w e d
a d ju s tm e n t to p H 3 .0 w ith H 2 S O 4 . D 0 filtra te w a s g e n e ra te d b y b le a c h in g s w e e tg u m
( k a p p a n o . 1 1 .8 ) w ith C lO 2 ( k a p p a f a c to r 0 .2 0 ) f o r 3 0 m in a t 4 5 o C
in a Q u a n tu m m ix e r (fin a l p H 2 .7 ).
2
P u lp in itia lly a t ~ 3 5 % c o n s is te n c y d ilu te d to 1 2 % c o n s is te n c y w ith D
0
filtra te , fo llo w e
a d ju s tm e n t to p H 3 .0 w ith H 2 S O 4 .
3
P u lp a c id ifie d to p H 4 a t ~ 2 % c o n s is te n c y a n d th ic k e n e d to ~ 2 5 % c o n s is te n c y p rio r to
d ilu tio n w ith D 0 filtra te to 4 % c o n s is te n c y .
4
P u lp in itia lly a t ~ 3 5 % c o n s is te n c y d ilu te d to 3 % c o n s is te n c y w ith 1 0 m M fo rm a te
b u ffe r, fo llo w e d b y a d ju s tm e n t to p H 3 .0 w ith H 2 S O 4 .
5
P u lp in itia lly a t ~ 3 5 % c o n s is te n c y d ilu te d to 3 % c o n s is te n c y w ith d ilu te a c id , p H 3 .0 .
T h e s u s p e n s io n w a s th e n th ic k e n e d to 1 2 % c o n s is te n c y .
Effect of Hydrolysis Consistency
Most laboratory studies of hydrolysis as a bleaching pretreatment have used low consistencies
for the acid treatment. On the mill scale, this is impractical because of the associated requirement
for large retention vessels, high filtrate recycle rates, and high steam consumption. The data of
Table 4.4 show the effect of increasing the consistency of the hydrolysis stage. The consistency
can be raised to 12% with no loss in efficiency or selectivity. Higher consistencies lead to poorer
selectivity and may also give slightly lower efficiency.
T a b le 4 .4 . E f f e c t s o f H y d r o ly s is C o n s is te n c y
U n b le a c h e d P u lp
1
T ype
K appa
N o.
V is c o s ity ,
m P a .s
C onv
1 2 .0
3 2 .9
P u lp in itia lly a t ~ 3 5 %
C o n s is 1
te n c y ,
%
4
12
20
30
K app a N o.
R e d u c tio n
o n
H y d ro ly s is
5
5
4
4
V is c o s ity
A fte r
H y d ro ly s is ,
m P a .s
.4
.2
.3
.9
c o n s is te n c y d ilu te d w ith D
fo llo w e d b y a d ju s tm e n t to p H 3 .0 w ith H 2 S O 4 . D
w a s g e n e ra te d a s in d ic a te d in p re v io u s ta b le s .
2
2
1
1
0
0
4
3
9
5
.4
.3
.5
.4
filtra te ,
filtra te
Effects of Prehydrolysis on Subsequent Delignification
Two conventional and three RDH sweetgum kraft pulps were delignified in the D0(EO) partial
bleaching sequence, with and without prior hydrolysis to remove hexenuronic acid. The resulting
data are shown in Table 4.5.
Table 4.5. Effect of Prehydrolysis on D0(EO) Delignification Efficiency
Unbl.Pulp
Hydrolysis
Before D0
Stage?
D0 Stage1
Kappa
Kappa
Number
Number
Entering Leaving
(EO) Stage2
NaOH
Charge,
% o.d.p.
Final
pH
DelignifiKappa Delignifi- cation by
Number cation, % D0(EO), %
D0(EO)
Delta
Kappa/
TAC
Pulp
Type
Kappa
Number
Conv.
11.4
No
Yes
11.4
7.0
6.2
4.1
2.5
1.1
12.4
11.6
4.2
2.0
32
51
63
71
3.16
3.57
15.2
No
Yes
15.2
9.5
7.9
5.9
2.0
1.3
12.0
11.8
5.2
2.2
34
63
66
77
3.29
3.84
7.8
No
Yes
7.8
5.6
5.6
3.6
1.7
1.0
11.9
11.5
3.1
1.8
45
50
60
68
3.01
3.39
13.4
No
Yes
13.4
8.8
10.0
5.4
1.7
1.0
12.0
11.0
5.2
2.3
48
57
61
74
3.06
3.69
22.9
No
Yes
22.9
15.4
13.9
10.8
1.7
1
11.6
10
5.2
3
63
72
77
81
3.86
4.03
RDH
1
D0 stage conditions: Quantum Mixer, 30 min., 45oC, 3% consistency, kappa factor 0.20, final pH 2.5-3.0.
2
(EO) stage conditions: Pin mixer, 60 min., 70oC, 10% consy., oxygen pressure initially 60psig and decreased
by 10 psig every 5 min.
One reason for obtaining the data shown in Table 4.5 was to verify that the reduction in kappa
number achieved in the hydrolysis step does indeed result in the expected corresponding
reduction in downstream bleaching chemical requirements. It is reasonable to expect, for
example, that the degree of delignification achieved in subsequent D0 and (EO) stages (expressed
as a percentage of the kappa number entering the D0 stage) should be no lower than in the
absence of prehydrolysis. The data of Table 4.5 not only verify this, but also reveal additional,
unexpected effects.
It is known that pulping to lower unbleached kappa numbers results in pulps that are more
difficult to bleach than those having higher kappa numbers, in the sense that they consume more
oxidant per unit of kappa number reduction. This phenomenon is apparent in the data of Table
4.5. Decreasing the unbleached kappa number of the conventional pulp from 15.2 to 11.4
decreased the delignification efficiency from 3.84 to 3.57 kappa units per unit of oxidant
consumption. Similarly, decreasing the unbleached kappa number of the RDH pulp from 22.9
to 7.8 decreased the delignification efficiency from 3.86 to 3.01. More relevant, however, is the
observation that prehydrolysis increases the efficiency of D0(EO) delignification in spite of the
fact that it decreases the kappa number of the pulp entering the D0 stage. The result is a sharp
increase in efficiency at any given entering kappa number, as illustrated in Figure 4.9.
One interpretation of this observation is that prehydrolysis conditions the residual lignin to make
it more easily removable. It may be speculated that this is due to hemicellulose removal,
breakage of bonds between lignin and carbohydrate, or beneficial changes in the structure of the
lignin itself. Alternatively, the improvement in efficiency may simply reflect the decrease in
hexenuronic acid content, if it is assumed that hexenuronic acid is more resistant than lignin to
removal by ClO2. This latter interpretation is, however, inconsistent with the observations by
Allison et al.64 and Törngren and Gellerstedt65 that hexenuronic acid are readily removed by
ClO2.
% Kappa Drop in D0(EO)
90
80
Prehydrolyzed
70
Untreated
60
Kappa Factor = 0.20
50
Conv.
RDH
Prehydr. Conv.
Prehydr. RDH
40
0
5
10
15
20
25
Unbleached Kappa Number
30
Figure 4.9. Effect of prehydrolysis on D0(EO) delignification.
Full Bleaching of Prehydrolyzed Pulps
A sweetgum conventional pulp having an unbleached kappa number of 12.6 was bleached in a
D(EO)DD sequence with and without a preceding hydrolysis step, giving the results shown in
Table 4.6. In bleaching the unhydrolyzed pulp to a brightness of 88.4, the total ClO2
consumption was 2.16%. Atmospheric prehydrolysis of the same pulp for 5 h at 95oC reduced its
kappa number to 6.7, allowing a brightness of 88.4 to be reached with only 1.71% ClO2. A
pressurized prehydrolysis for 1 h at approximately 125oC reduced the kappa number to 6.1 and
allowed bleaching to 88.7 brightness with the expenditure of only 1.26% ClO2. Finally, the same
sequence was repeated, except that the pulp was transferred directly from the prehydrolysis to
the D0 stage with no intermediate washing.
Table 4.6. D(EO)DD Bleaching After Hydrolysis
D0 Stage2
HydroWash
lysis1
Before Kappa After
Number PreD0
Stage Enter- hydroing
lysis?
None
Atm.
Press.
Press.
12.6
6.7
6.1
6.1
Yes
Yes
Yes
No
(EO) Stage3
D1 Stage4
D2 Stage4
Viscosity Kappa
ClO2, %
Enter- Number NaOH
Visc- ClO2, %
o.d.
Brighto.d.
Brighting,
Leav- Charge, Kappa osity,
pulp
ness
pulp
ness
mPa.s
ing % o.d.p. Number mPa.s
34.8
22.4
14.6
22.1
5.7
3.4
2.3
4.4
1.0
1.0
1.0
1.3
3.5
1.7
1.5
2.2
33.2
21.1
13.7
21.2
0.8
0.4
0.4
0.8
84.0
84.4
85.7
84.8
0.4
0.8
0.4
0.6
88.4
88.4
88.7
88.7
Summary
Viscosity,
mPa.s
25.1
16.1
13.3
n.d.
Total
No. of ClO2, %
Washo.d.
ers
pulp
4
5
5
4
2.16
1.71
1.26
1.86
1
Hydrolysis conditions: 12% consistency; initial pH adj. to 3.0 with H2SO4. Atmospheric: 5 h, 95oC; Pressurized: 1 h, approx. 125oC
2
D0 stage conditions: 30 min., 45oC, 3% consistency, final pH 2.5-3.0.
3
(EO) stage conditions: Pin mixer, 60 min., 75oC, 10% consy., oxygen pressure initially 60 psig and decreased
by 10 psig every 5 min.
4
D1 and D2 stage conditions: 180 min., 70oC, 10% consistency, final pH 3.7-4.5.
In this case, 1.86% ClO2 was required to reach 88.7 brightness. Although no optimization
was attempted in these experiments, the results demonstrate the potential for
commercially viable applications of prehydrolysis in the bleaching of southern HW
pulps. Significant ClO2 savings may be realizable, especially if a washer is available for
use after the prehydrolysis stage. With no washer at this point ClO2 savings will likely be
smaller, but still appreciable.
Residual Lignin Studies
The bleachability of HW kraft pulps can be influenced by both the presence of
hexenuronic acids and the structure of their residual lignin. Past studies by Gellerstedt,
Froass et al., Argyropoulos et al., and others48,70,66 have clearly established the role of
lignin in influencing pulp bleachability for SW kraft pulps. To explore the relationship
between bleachability, lignin structure, and pulping technology, the HW kraft pulps
prepared for this study were analyzed for residual lignin structure. These structure
studies were accomplished by isolating lignin from the RDH and conventional kraft pulps
using an acidic dioxane lignin extraction procedure. The residual lignin samples from the
kraft brownstocks were then analyzed by quantitative 13C NMR methods. This procedure
provides a facile means of determining the relative amounts of various lignin structural
components including diphenyl methane units, methoxyl groups, β-O-aryl ether units,
and acid groups. Figures 4.10- 4.13 summarize the results.
0.45
Relative amounts of Acid Groups
0.4
0.35
0.3
Conventional
RDH
0.25
0.2
0.15
0.1
0.05
0
10
12
14
27
7
13
14
22
Pulp Kappa #
Figure 4.10. Residual lignin acid group content for series of conventional and RDH
pulps. (Note: acid group content is relative to signal intensity of aromatic signal).
1.4
Conventional
RDH
Relative amounts of MeO
1.2
1
0.8
0.6
0.4
0.2
0
10
12
14
27
7
13
14
22
Figure 4.11. Residual lignin methoxyl group content for series of conventional and
RDH pulps. (Note: MeO content is relative to signal intensity of aromatic signal).
0.6
Relative Amounts of CH2Ph2
0.5
0.4
Conventional
0.3
RDH
0.2
0.1
0
10
12
14
27
7
13
14
22
Pulp Kappa #
Figure 4.12. Residual lignin diphenyl methane group content for series of
conventional and RDH pulps. (Note: Ph2C content is relative to signal intensity of
aromatic signal).
In general, the results are consistent with modern theories of pulping chemistry. They
indicate that the lignin suffers a depletion of β-O-aryl ether units and methoxyl units as
the kappa number of the pulp is reduced from 27 to 7. In addition, as the kraft cook is
extended, the residual lignin in the pulp becomes enriched in acid groups and condensed
lignin structures. Of greater significance in the context of the present study is the very
close structural resemblance between the RDH and conventional kraft pulps. This
similarity exists in spite of the differences between the two batch cooking technologies
(RDH and conventional) employed in the study. It suggests that lignin structure is
unlikely to result in differences in bleachability at a given kappa number.
Relative amounts of Beta-O-aryl ethers
0.5
0.45
Conventional
RDH
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
10
12
14
27
7
13
14
22
Pulp Kappa #
Figure 4.13 Residual lignin β-O-aryl ether group content for series of conventional
and RDH pulps. (Note: β-O-aryl ether content is relative to signal intensity of
aromatic signal).
4.3 Conclusions:
D0 stage filtrate, pH 3 formate buffer, and sulfuric acid give similar results when used as
media in acid hydrolysis to reduce the kappa number of HW kraft pulps. There are no
significant differences between conventional and simulated RDH pulps with respect to
their response to hydrolysis.
Under atmospheric conditions, complete hydrolysis, as indicated by kappa number
reduction, requires a retention time of approximately 5-hours. On the other hand, it is
possible to achieve appreciable extents of removal at retention times in the 2-3 h range.
Pressurized hydrolysis for 2 h at 100oC reduces the kappa number of HW pulps to the
same extent as atmospheric hydrolysis for 5 h at 95oC. Increasing the temperature to
125oC allows the retention time to be shortened to 1 h with no loss in removal efficiency,
or to fifteen minutes with a 20% loss in efficiency. Viscosity losses were modest under
all conditions.
In atmospheric hydrolysis, the consistency can be raised to 12% with no loss in
efficiency or selectivity. Higher consistencies lead to poorer selectivity and may also give
slightly lower efficiency. Prehydrolysis of HW kraft pulps increases the efficiency of
subsequent D0(EO) delignification in spite of the fact that it decreases the kappa number
of the pulp entering the D0 stage. The result is a sharp increase in efficiency at any given
entering kappa number.
The potential exists for commercially viable applications of prehydrolysis in the
bleaching of southern HW pulps. Significant ClO2 savings may be realizable, especially
if a washer is available for use after the prehydrolysis stage. With no washer at this point,
ClO2 savings will likely be smaller, but still appreciable.
Residual lignins in pulps prepared in the laboratory by conventional batch kraft cooking
and simulated RDH cooking are structurally very similar. This observation is consistent
with their similar response to D(EO) delignification.
4.4 Experimental:
Materials
All chemical reagents were commercially purchased and used as received. Commercial
HW and SW kraft pulps were acquired from several mills, principally in the US
southeast. In addition, a series of SW and HW batch and extended modified kraft cooks
were performed in the laboratory. The SW kraft laboratory cooks were described in
chapter 2 of this report and originated from a sole loblolly pine wood source. All pulps
employed for hexenuronic acid analysis were extensively washed until the effluents were
pH neutral and colorless.
HW Kraft Pulping
All kraft cooks were performed using a common supply of sweetgum from a 20–25 yearold tree. The wood was visually free of disease and compression wood. After debarking
and chipping, the chips were screened and stored at 4oC until used. Conventional HW
kraft cooking trials67 were performed at a 4:1 liquor/wood ratio using 30% white liquor
sulfidity (active alkali basis). Chips and cooking liquor were heated to 165oC for 90
minutes and kept at that temperature until the desired H-factor was reached.
Rapid Displacement Heating (RDH) kraft cooking trials68 were performed using a fully
automated control system employing white liquor with 30% sulfidity (active alkali basis)
and a cooking temperature of 165oC. White liquor addition was split into warm and hot
black liquor pretreatments. The wood chips were pretreated with warm and hot black
liquors before cooking with white liquor. At the end of the cook, hot cooking liquor
inside the digester was displaced back to the hot and the warm black liquor accumulators
using washer filtrate from a brownstock washing system. Table 4.1 summarizes some of
the important HW kraft cooking parameters and pulp properties.
Bleaching
Conditions for each bleaching stage are given in footnotes to the bleaching data tables.
Washing was omitted after the hydrolysis stages. Otherwise, pulps were well washed
after each stage. General descriptions of the individual stages are given below.
Hydrolysis stages were done in plastic bags placed either in a temperature-controlled
water bath or in a glass beaker over water in an autoclave. Initial chlorine dioxide (D0)
stages were done in a Quantum mixer or in plastic bags. Simple extraction (E2) stages as
well as D1 and D2 stages were done in sealed plastic bags in a temperature-controlled
water bath. Oxygen-reinforced extraction (EO) stages were done in a pressurized pin
mixer with electric heating.
Pulp Testing
All pulp testing was done according to TAPPI Test Methods.
Xylan Analysis of Pulps
The xylan content of selected kraft pulps was determined following literature
methods.69
Residual Lignin Isolation and Analysis
Isolation of residual lignin from the HW kraft pulps was accomplished by employing
standard literature methods.48,70,71 In brief, air-dried pulp (40 g oven dry weight) was
added to an aqueous 1.00 N HCl (100 ml), p-dioxane (900 mL) solution and this mixture
was refluxed for 2 hr under an argon atmosphere. The mixture was then filtered, pH
adjusted to 5 with a saturated NaHCO3 aqueous solution, and vacuum concentrated to
10% of its initial volume. The solution was then pH adjusted to 2.0 with 1N HCl, and the
precipitated lignin was isolated by centrifugation. The lignin was then washed and
vacuum dried. This procedure afforded, on average, a 40-55% yield of residual lignin in
the pulp.
13
C NMR Analysis of Residual Lignin
Quantitative 13C NMR spectra were recorded with an inverse gated 90o pulse sequence, a
14 s delay, a TD of 32k, and a sweep width of 230 ppm.72 The NMR experiments were
performed at 50°C on samples containing 200-400 mg lignin/mL of DMSO-d6. The
Fourier transformed spectra were integrated according to reported chemical shifts for
lignin functional groups. The integrals were normalized to the aromatic signals, which
were assumed to have 6 carbons.
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